Only one group can be the direct owner of a directory, file, device, etc. However, the members of a group can be other groups. For example, on one of my boxes I have CD burners on /dev/hdc and /dev/hdd. The group owning them is cdrom. The members of cdrom are the group disk and 3 of my users.
Your permissions are fixed across the groups though. The rw permissions I have assigned to cdrom carry through to all members and can't be changed for a subset. Since there are three permission groups (user/owner, group, other/world) you may be able to achieve what you need with the 'user' or 'other' permissions.