Hi,
The group should be a group from theUnix group database of the filer, not from any Unix server you are mounting from, and not an Active Directory group.
In its simplest form, you should be able to add entries to /etc/group on the storage system.
By default, you only have something like this in it:
fas3240-tcb1*> rdfile /etc/group
daemon:*:1:
I believe it should also be possible to use NIS and/or LDAP to store this group information in case you need to keep track of the group <-> groupID mappings centrally.