Was it something that was deleted accidentally as the admin? the most effective way is to restore to the last backup (and then make changes from that point again) you can go through the admin access log and see when the users were added and do it again
If you look at the admin access log, you will see entries for each of the users that were deleted such as "User Accounts modified. Removed username from authentication server ."
I re-read your post and realized I may have answered the wrong question, if I did, I apologize You should also be able to see a message regarding the removal. Depending on how the deletion happened, you should see one of two messages:
Realm : Deleted User Name role mapping rule '': user = ('user' or 'user1' or 'user2' or 'user3' or 'user4' or 'user5') for when the usernames were removed