BuddyPress.org

For your first problem there is no plugin available.. If you want to display activity/group/forum stream on a different blog then your main blog, you might need to make custom templates for that blog.. You also might need to have some custom code written, but I’m not sure, maybe an expert could help on that point..

What do you mean exactly with managing memberships? If you want to charge your members and offer different member types, S2Member is a great solution to look into. http://www.s2member.com.