I can't create users. I updated from 12.10 to 13.04 and only see one user, the two others are gone, I upgraded keeping my files.
However, using the command line to do this task doesn't explain why you can't use the GUI. Stupid question here, but have you clicked on the "unlock" icon at the upper right of the users GUI to bring up the "authenticate" window? (I've had my own share of Doh moments!)
You might backup your user data as well as that of the other users before you do anything else. You should already have done this before you did the distribution upgrade anyway.