this doesn't have anything to do with perl, but note that you can have multiple lines for the same group in an /etc/group file. ie, these two are equivilent:

Case 1:
admin::111:bob,joe,fred

Case 2:
admin::111:bob,joe
admin::111:fred

of course, that is only if you are editting your group file by hand. also note that there is typically a limit to the number of users that you can have in a single group.