store objects with a custom attribute

Hi,

i try to store objects which differ through a complicated relationship. So e.g. i want to store the moderators, the user ... of another object. Currently i create an array for each relationship where i store the users with this particular relationship (@members_users, @members_moderators, ....). I think thats not the best way, so can i add a custom attribute just for this case and not in the db to differ between the different relationships and that i can store all members in one array? Do you've got another idea to handle this situation?