Yes. Since Group IDs (gid) are valid on home and nest volumes, users can share their files among group members by setting appropriate permissions. The followings are the examples of the instruction.
$ chgrp (GROUP-ID) (HOME) ex．chgrp t2ggroup user-a-aa
$ chgrp (GROUP-ID) (SHARE) ex．chgrp t2ggroup user-a-aa/share
$ chmod 710 (HOME) ex．chmod 710 user-a-aa
$ chmod 770 (SHARE) ex．chmod 770 user-a-aa/share
$ chmod g+s (SHARE) ex．chmod g+s user-a-aa/share
(HOME) : Path to home directory
(GROUP-ID): TSUBAME group
(SHARE): Path to shared directory
By setting appropriate permissions of files on your home directory, you can restrict the accesses from other group members.
ex. 755 -> 700
Note that each user is allowed to use home and nest2 volumes up to 25GB respectively and the amount of the storage usage is distinguished by owner information of files (uid); therefore, even if a user writes files to other user's directory, the files are included in owner's storage usage .