The 28-groups limit is caused by the fact that SME adds users 'admin' and 'www' to each group it creates. If you use the --no-admin switch with lat-groups, the group will be created without those users, effectively allowing you to create more than 28 groups.
Please note, however, that SME is very persistent with this 'feature'. Each time you use the server-manager to modify a group, 'admin' and 'www' will be added to all groups again. So if you add more than 28 groups to your server, you better no longer use the 'groups' option in the server-manager.
Use it at your own risk! I've warned you...
Michiel