I am helping a customer setup BuddyPress / bbpress on a multisite. They have 3 conferences and are wanting to have a site for each conference with different users for each conference/site. They have additional profile fields defined under the network setup/users. When I register on the main site (default site that was created when I installed MS), I see those fields. When I register on a site that I have created, those fields do not show. I can send the url to the sites separately if that helps.