With the latest update, it looks like it just displays the user's Display Name, but that's not unique and can be confusing if multiple users have similar (or even the same) Display Name.
It should either show the username in parenthesis after the Display Name so users can be uniquely identified.
Would also be nice if you could enter a display name and have it auto-complete like it does for assigning tickets.
Diff:
created #92: [#4169] Show user names on permissions page (1cp)
Related
Tickets:
#4169closed #92, see branch 42cc_4169