"Community garden" isn't quite the right term. It's actually primarily a food rescue organisation (takes unsold produce etc from supermarkets and redistributes it to poor people) that also grows a bit of their own produce as well, which is the bit I help with.

I happened to be at a kind of mini festival where there was a stand actively recruiting for volunteers. They seemed pretty surprised and happy when I immediately said yes.

The actual "community gardens" that I know of generally are on the model of members each have their own plot, which they tend. I don't know how one volunteers in a space like that, but maybe there are opportunities.