Honestly, I was looking at Buddypress at this very moment. It seems a lot of complexity but at least you have plenty of documentation and resources (maybe too much). I’ll give ultra community another day before opting out.
All I need is to have a way to group users (non profit organizations in this case) in themes so a visitor looking for “animal welfare orgs”, “human rights”, or “fighting poverty”, for example, will find a list with organizations he/she may help. Each organization would have a profile page with its activities, volunteer opportunities, contact info, how to donate, etc.