Just go to them and eventually you'll have something to bring back. The varieties come and go so it's helpful if you bring back the same variety after a few years.
Membership is not required for most of them but they encourage you to join and go to the meetings. Some will charge a small fee for non-members or set up a donation box. The OC one has people bringing home made and purchased food for breakfast. You could bring an item if you're good at cooking. They're just laid out on a table and people walk by and take a couple of things. Same with the scions.
You don't need to join before going to the meetings. Go a couple of times before you decide.