My wife & I are Scottish Country dancers. It's a social dance form (it's traditional to swap partners for every dance so nobody has to bring a partner, though not required). Pretty good cardio, there are groups all over the world, so it's often not that difficult to find a class. Other similar social dance forms from the UK are Contra dancing, English Country dancing, and Ceilidh dancing. Square dancing in the US dev…
> Meaningful contribution is easy: these groups always benefit from more participants.
Same for probably all small dance communities! While it seems like a different kind of 'contribution' than the OP is looking for, it's very meaningful personally to share music and creativity with people. (Personally, I dance lots of Balboa -- a swing dance with a local scene of probably less than 100 active participants.)