Unable to add users to team using the @mention feature

Issue Summary

When you try to add some users to teams using the @mention feature, it displays nothing. This doesn't affect all users, I was able to find some users using the @mention on my test instance.

Steps to Reproduce

1. Head over to People page
2. Create a new team and click on Add People
3. Look for a user using the @mention feature

Expected Results

The users appear in the drop-down list as a result

Actual Results

Returns nothing instead

Workaround

Find the user name without using the @mention feature.