can non-LDAP groups be shared across sites?

hi,

i've several distinct sites and a number of distinct (non-LDAP) groups.

i want to permission some groups to be able to access multiple sites.

is this possible?

site administrators are shared across sites.

but i can't see how to share the other groups (and users) i've created.

i can create copies of groups in multiple sites. 

but i can't allocate users across sites (so same problem).

i can create copies of users across sites. 

but although they share username/password they are distinct DB entities.

ideally i'd like to avoid (different "my settings", additional admin etc).

migrating to LDAP is not really a viable option at this point in time.

any pointers would be much appreciated. 

cheers,

paul.