fix: issue with group users having the same name

[tynanmatthews]

Checklist

• I'm using the correct PHP Version (7.4 for current, 7.2 for legacy).
• I've added comments to any new methods I've created or where else relevant.
• I've replaced magic method usage on DbService classes with the getInstance() static method.
• I've written any documentation for new features or where else relevant in the docs repo.

Description

Fixed an issue where users with the same name would not be shown in the list when attempting to add new members.
This was done by changing the key from full name to user ID when fetching the list of users.
Also added email addresses to differentiate users with the same name.

Changelog

refs:
issues: #207

Other Information

Docs pull request:

[chris-bateman]

@tynanmatthews could you look at this and work with @DaltJM on deploying it?