A user cannot be a member of more than 32 groups. This is a Linux thing. (can be changed but...)
In SME, 'admin' and 'www' are members of every group you make, probbably because of rights to access and change the groups made via the server-manager.
A problem arises when you create more than 28 groups in sme (sme creates 4 groups itself which you cannot change) because of the 32-limit. Admin cannot access the server-manager anymore because of admin being removed from the group which has access to server-manager.
It is possible however to create an almost unlimited number of groups within Linux.
What I am trying to accomplish is creating groups without admin in a simple way. This would be a workaround for this 'flaw' in SME. These groups cannot be changed through server-manager of course.
More info:
http://forums.contribs.org/index.php?topic=7792.msg28760#msg28760 This 'bug' was first reported in 4.1.2 and solved in 5.0 by not being able to create more than 28 groups via server-manager.