Having added the user, do they have a /home directory entry? If they do, then likely the control center is running the useradd command correctly (I've had no real issue with it under 9.2, anyway), and might be related to needing an update for some other piece of MDK to handle the "first time login" actions for a new user. (I believe 9.2 had this issue out-of-the-box, but an update did alleviate the problem.)
Can you also switch to a virtual terminal (i.e., use ctrl-alt-f2) and login that way as one of the new users? Great thing to check. (use ctrl-alt-f7 to get back to the normal X session, btw).