This problem is rather rare. And especially because it is rare, when it comes, you scratch your had a bit.
See some solutions below.

The error:

gzaix# mkuser lame
3004-691 Error changing "groups".

The solution (rather the cause):

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 ;)