diff --git a/src/generic/processing-notification/ProcessingNotification.test.jsx b/src/generic/processing-notification/ProcessingNotification.test.jsx
index 97f57429bf..d2bbdbeaca 100644
--- a/src/generic/processing-notification/ProcessingNotification.test.jsx
+++ b/src/generic/processing-notification/ProcessingNotification.test.jsx
@@ -1,13 +1,11 @@
-import { capitalize } from 'lodash';
 import userEvent from '@testing-library/user-event';
 import { initializeMocks, render, screen } from '../../testUtils';
-import { NOTIFICATION_MESSAGES } from '../../constants';
 import ProcessingNotification from '.';
 
 const mockUndo = jest.fn();
 
 const props = {
-  title: NOTIFICATION_MESSAGES.saving,
+  title: 'ThIs IS a Test. OK?',
   isShow: true,
   action: {
     label: 'Undo',
@@ -22,16 +20,16 @@ describe('<ProcessingNotification />', () => {
 
   it('renders successfully', () => {
     render(<ProcessingNotification {...props} close={() => {}} />);
-    expect(screen.getByText(capitalize(props.title))).toBeInTheDocument();
+    expect(screen.getByText(props.title)).toBeInTheDocument();
     expect(screen.getByText('Undo')).toBeInTheDocument();
     expect(screen.getByRole('alert').querySelector('.processing-notification-hide-close-button')).not.toBeInTheDocument();
     userEvent.click(screen.getByText('Undo'));
-    expect(mockUndo).toBeCalled();
+    expect(mockUndo).toHaveBeenCalled();
   });
 
   it('add hide-close-button class if no close action is passed', () => {
     render(<ProcessingNotification {...props} />);
-    expect(screen.getByText(capitalize(props.title))).toBeInTheDocument();
+    expect(screen.getByText(props.title)).toBeInTheDocument();
     expect(screen.getByRole('alert').querySelector('.processing-notification-hide-close-button')).toBeInTheDocument();
   });
 });
diff --git a/src/generic/processing-notification/index.jsx b/src/generic/processing-notification/index.jsx
index b31150a957..42dc95711f 100644
--- a/src/generic/processing-notification/index.jsx
+++ b/src/generic/processing-notification/index.jsx
@@ -3,7 +3,6 @@ import {
   Icon, Toast,
 } from '@openedx/paragon';
 import { Settings as IconSettings } from '@openedx/paragon/icons';
-import { capitalize } from 'lodash';
 import classNames from 'classnames';
 
 const ProcessingNotification = ({
@@ -18,7 +17,7 @@ const ProcessingNotification = ({
   >
     <span className="d-flex align-items-center">
       <Icon className="processing-notification-icon mb-0 mr-2" src={IconSettings} />
-      <span className="font-weight-bold h4 mb-0 text-white">{capitalize(title)}</span>
+      <span className="font-weight-bold h4 mb-0 text-white">{title}</span>
     </span>
   </Toast>
 );
diff --git a/src/generic/toast-context/index.test.tsx b/src/generic/toast-context/index.test.tsx
index f7e0a2e4b0..11294b0699 100644
--- a/src/generic/toast-context/index.test.tsx
+++ b/src/generic/toast-context/index.test.tsx
@@ -13,7 +13,7 @@ const TestComponentToShow = () => {
   const { showToast } = React.useContext(ToastContext);
 
   React.useEffect(() => {
-    showToast('This is the toast!');
+    showToast('This is the Toast!');
   }, [showToast]);
 
   return <div>Content</div>;
@@ -23,7 +23,7 @@ const TestComponentToClose = () => {
   const { showToast, closeToast } = React.useContext(ToastContext);
 
   React.useEffect(() => {
-    showToast('This is the toast!');
+    showToast('This is the Toast!');
     closeToast();
   }, [showToast]);
 
@@ -59,19 +59,19 @@ describe('<ToastProvider />', () => {
 
   it('should show toast', async () => {
     render(<RootWrapper><TestComponentToShow /></RootWrapper>);
-    expect(await screen.findByText('This is the toast!')).toBeInTheDocument();
+    expect(await screen.findByText('This is the Toast!')).toBeInTheDocument();
   });
 
   it('should close toast after 5000ms', async () => {
     render(<RootWrapper><TestComponentToShow /></RootWrapper>);
-    expect(await screen.findByText('This is the toast!')).toBeInTheDocument();
+    expect(await screen.findByText('This is the Toast!')).toBeInTheDocument();
     jest.advanceTimersByTime(6000);
-    expect(screen.queryByText('This is the toast!')).not.toBeInTheDocument();
+    expect(screen.queryByText('This is the Toast!')).not.toBeInTheDocument();
   });
 
   it('should close toast', async () => {
     render(<RootWrapper><TestComponentToClose /></RootWrapper>);
     expect(await screen.findByText('Content')).toBeInTheDocument();
-    expect(screen.queryByText('This is the toast!')).not.toBeInTheDocument();
+    expect(screen.queryByText('This is the Toast!')).not.toBeInTheDocument();
   });
 });
diff --git a/src/library-authoring/library-team/LibraryTeam.test.tsx b/src/library-authoring/library-team/LibraryTeam.test.tsx
index 2ab5ec52ab..bdd424fc08 100644
--- a/src/library-authoring/library-team/LibraryTeam.test.tsx
+++ b/src/library-authoring/library-team/LibraryTeam.test.tsx
@@ -15,6 +15,7 @@ import {
   getLibraryTeamMemberApiUrl,
 } from '../data/api';
 import { LibraryProvider } from '../common/context';
+import { ToastProvider } from '../../generic/toast-context';
 import LibraryTeam from './LibraryTeam';
 
 mockContentLibrary.applyMock();
@@ -28,9 +29,11 @@ describe('<LibraryTeam />', () => {
   const { libraryId } = mockContentLibrary;
   const renderLibraryTeam = async () => {
     render(
-      <LibraryProvider libraryId={libraryId}>
-        <LibraryTeam />
-      </LibraryProvider>,
+      <ToastProvider>
+        <LibraryProvider libraryId={libraryId}>
+          <LibraryTeam />
+        </LibraryProvider>
+      </ToastProvider>,
     );
 
     await waitFor(() => {
@@ -176,6 +179,56 @@ describe('<LibraryTeam />', () => {
         `{"library_id":"${libraryId}","email":"another@user.tld","access_level":"read"}`,
       );
     });
+
+    expect(await screen.findByText('Team Member added')).toBeInTheDocument();
+  });
+
+  it('shows error when user do not exist', async () => {
+    const url = getLibraryTeamApiUrl(libraryId);
+    const axiosMock = new MockAdapter(getAuthenticatedHttpClient());
+    axiosMock.onPost(url).reply(400, { email: 'Error' });
+
+    await renderLibraryTeam();
+
+    const addButton = screen.getByRole('button', { name: 'New team member' });
+    userEvent.click(addButton);
+    const emailInput = screen.getByRole('textbox', { name: 'User\'s email address' });
+    userEvent.click(emailInput);
+    userEvent.type(emailInput, 'another@user.tld');
+
+    const saveButton = screen.getByRole('button', { name: /add member/i });
+    userEvent.click(saveButton);
+
+    await waitFor(() => {
+      expect(axiosMock.history.post.length).toEqual(1);
+    });
+
+    expect(await screen.findByText(
+      'Error adding Team Member. Please verify that the email is correct and belongs to a registered user.',
+    )).toBeInTheDocument();
+  });
+
+  it('shows error', async () => {
+    const url = getLibraryTeamApiUrl(libraryId);
+    const axiosMock = new MockAdapter(getAuthenticatedHttpClient());
+    axiosMock.onPost(url).reply(400, {});
+
+    await renderLibraryTeam();
+
+    const addButton = screen.getByRole('button', { name: 'New team member' });
+    userEvent.click(addButton);
+    const emailInput = screen.getByRole('textbox', { name: 'User\'s email address' });
+    userEvent.click(emailInput);
+    userEvent.type(emailInput, 'another@user.tld');
+
+    const saveButton = screen.getByRole('button', { name: /add member/i });
+    userEvent.click(saveButton);
+
+    await waitFor(() => {
+      expect(axiosMock.history.post.length).toEqual(1);
+    });
+
+    expect(await screen.findByText('Error adding Team Member')).toBeInTheDocument();
   });
 
   it('allows library team member roles to be changed', async () => {
diff --git a/src/library-authoring/library-team/LibraryTeam.tsx b/src/library-authoring/library-team/LibraryTeam.tsx
index b41be5acf2..8bf4d2e31c 100644
--- a/src/library-authoring/library-team/LibraryTeam.tsx
+++ b/src/library-authoring/library-team/LibraryTeam.tsx
@@ -65,8 +65,13 @@ const LibraryTeam: React.FC<Record<never, never>> = () => {
         accessLevel: LibraryRole.Reader.toString() as LibraryAccessLevel,
       }).then(() => {
         showToast(intl.formatMessage(messages.addMemberSuccess));
-      }).catch(() => {
-        showToast(intl.formatMessage(messages.addMemberError));
+      }).catch((addMemberError) => {
+        const errorData = typeof addMemberError === 'object' ? addMemberError.response?.data : undefined;
+        if (errorData && 'email' in errorData) {
+          showToast(intl.formatMessage(messages.addMemberEmailError));
+        } else {
+          showToast(intl.formatMessage(messages.addMemberError));
+        }
       });
       closeAddLibraryTeamMember();
     },
diff --git a/src/library-authoring/library-team/messages.ts b/src/library-authoring/library-team/messages.ts
index 6bf6a8c363..d56d606153 100644
--- a/src/library-authoring/library-team/messages.ts
+++ b/src/library-authoring/library-team/messages.ts
@@ -124,6 +124,11 @@ const messages = defineMessages({
     defaultMessage: 'Error adding Team Member',
     description: 'Message shown when an error occurs while adding a Library Team member',
   },
+  addMemberEmailError: {
+    id: 'course-authoring.library-authoring.library-team.add-member-email-error',
+    defaultMessage: 'Error adding Team Member. Please verify that the email is correct and belongs to a registered user.',
+    description: 'Message shown when an error occurs with email while adding a Library Team member.',
+  },
   deleteMemberSuccess: {
     id: 'course-authoring.library-authoring.library-team.delete-member-success',
     defaultMessage: 'Team Member deleted',