Adding new coordinators in manage-coordinators page

[justinmend]

When an admin is trying to add a new coordinator with a username that is already taken, should we display a message indicating why it is not allowing them to add that coordinator with the same username? So far nothing will happen if the admin tries to add a coordinator with the same username.

[ozerugae]

No longer required. Only already created users can be added. User will have to sign in first before being added