This problem is rather rare. And especially because it is rare, when it comes, you scratch your had a bit.
See some solutions below.
gzaix# mkuser lame 3004-691 Error changing "groups".
This error can appear due to errors in /etc/group file so look especially for:
* missing : - for example, a group without users should have the line ended with :
* blank line - yes, if you have empty line in /etc/group file, you can get this error also.
Will add others if appear ;)