AAA Database

Hi All, I have one question about the AAA database. In AAA server whether the entries will be stored in a per user based? i.e. seraching the entries on the NAI base? Can we store the entries on the group of users and have common authentication for the users. I think its possibile only if we have same username and passward used by all the users. Is there any authentication based on the subnet? Thanks in advance. Best Regards, Wable