Please note there is a shortcoming in how the users are displayed in the basic "user" field in MetaBox.
When the users are listed and 2 users have the same first name or display name selected, there is no way to tell them apart!
I would recommend to list all users as: [First-Name] [Last-Name] ([username])
As an alternative, you could allow to customise how the users are shown in this field through its definition. I could not find any way to do so in the docs, so if this is already feasible please let me know how 🙂