Don't know about the name, but since you can get the group number via something like this:
if ($wb->get_session('GROUP_ID')==1)
I suspect you will need to use the number to look it up in the GROUPS array.
See
http://projects.websitebaker.org/websitebaker2/wiki/Docs-EN-Variables and look for GROUPS (there are several references). You can check the Group administration page to find out how it populates the list there and do something similar.