Just use the forum itself, that's what it's here for. There already are a few "Mmebers from X area" threads. If you want to start a thread feel free to start one in the discussion forum. Just use topic like "Maine co-op" or "Eastern Main Co-op" or whatever you prefer. If enough get stated from various areas I'll make a sticky with links to the various ones so they are easier to find.
You could "advertise" it yourself by adding a link to the yopic in your signature. That's attached to every post you make.