It seems to me that the only way to add or remove users, or to change permissions, is to use directly modify the users table with phpMyAdmin, or something like it, or the MySQL command line.
I'm okay with that, but my client is not. I think most users would like some way to make such changes within the application.
I'm sure there is no shortage of possible enhancements that could be made. Just my $0.02 in case you needed one more recommendation.