DB access, functions to list sites, users and BP-groups.

I have tried to get some help with this over at the Buddypress forums, but perhaps it can be done using WP functions and knowledge.

I have a WPMS site on which I am running buddypress. So I have 3 “objects”, users, sites, and groups (groups are buddypress). I am writing a theme template page in which I wish to collate some data about the users and the sites, regardless of the current logged-in user or visited page.

E.G I’d like a function that listed the sites a user belonged to, or the users of a group. Do any such functions exist, and if so where can I find the documentation to show me how to use them?
If they do not exist, are there any issues in including SQL calls directly from functions.php in the theme folder?

Many thanks,
Andrew