John James Jacoby, Lead developer of BuddyPress said this.
BuddyPress sits on top of an entire WordPress
installation, regardless of configuration. That means on single-site,
multi-site, or multi-network installations, BuddyPress only has 1 set
of data tables. As a result, you only have 1 set of Groups, Activity,
Private Messages, Profiles, and Friends, even if you have 1 site/1
network, 100 sites/1 network, 100 sites per network/100 networks.
BuddyPress comes with a global const setting called
"BP_ENABLE_MULTIBLOG" that allows for all of the BuddyPress template
pieces to be available from any site on your installation where the
theme supports it by having the appropriate template files in place.
I.E. - network = domain1.com site = sub1.domain1.com
...will display the same groups as...
... with the only difference being the theme that is displaying them.
Since BuddyPress 1.2.6, the BuddyPress table names include filters
that in theory allow for breaking out of this 'global' arrangement,
however no site that I've seen has used it yet.