Does having BuddyPress installed on a Multisite setup mean that users in each of the different sites can interact or view the actions or content of users on other sites in the network?
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.