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 developed out of these forms. Many other cultures have their own social dance forms, with varying levels of formalization.

Meaningful contribution is easy: these groups always benefit from more participants. Scottish Country dance has a formalized teaching certificate program, roughly equivalent to a Master's degree worth of work (and if you're a UK resident it qualifies to teach PE in UK schools).