@symbols are your standard way of expressing usernames, but fail in all of your search fields
Summary
@chromakey does not appear as a user when trying to add a user in a group or project. "chromakey" does.
Steps to reproduce
Type @chromakey in a search field and watch in absurd disbelief that a user you know exists, does not appear.
Example Project
All of them.
What is the current bug behavior?
Your search should just remove the @ when sanitizing user input if it can't handle it, while still displaying to the user the appropriate @chromakey in the search field.
What is the expected correct behavior?
Searches with @ included should show users. When you search without the @, users come up with @ in front of their username. It's like you hired someone to make things confusing and difficult and he did a pro job at it.
Possible fixes
When sanitizing user input escape the @. Leave it alone in the UI. Job done, it should take 5 minutes.