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?