Are you talking about the Sno-Isle co-op store in Everett? You don't have to join to shop there - its just like a regular small natural foods grocery store. Maybe you are thinking of something else though???? I'm not sure what benefits, if any, you get through joining the co-op. Except that you are helping to support the co-op existing in the first place. I know at PCC you get a monthly 10% off coupon, but I'm not sure if you get any other monetary benefit to being a member. HTH!
I think you get 10% of all your purchases. I'm not a member because I only go once a month or so, so it wouldn't really be worth it for me. I can't remember how much it was per year, maybe $50? Don't quote me on that, though! It doesn't say on their <a href="http://www.snoislefoods.coop/Default.aspx" target="_blank">website</a>. Like Mamasong said, you don't have to be a member to shop there and utilize the sales either.