multiple mu installs using one database.

I was wondering what you guys thought of the idea of setting up several wordpress mu/buddypress sites with a shared database. Essentially Im trying to create several mu installs that can function as entry portals to one shared pool of users and blogs. I've been reading about some of the options but am not particularly clear as to the direction to take. I understand that to avoid blog id overlaps/conflicts usernames registered at one mu install would also register at all installs due to the shared database. this is fine, avoiding obsticals is good.. basicly, is their anyway for several wordpress mu installs to share a database? or a better way to accomplish several entry portals to one wordpress mu install?