My first co-op was Honest Weight in Albany, NY as a youngster. Since then I've been a member of Putney Co-op, worked at the now defunct Good Day Market in Portland, ME, and am a current member/former employee of Wheatsville.
Fun fact: My college career was partly paid for by a series of scholarships from the local grocery chain H.E.B. for "individuals pursuing a career in the grocery industry". They didn't seem to care that my interest was in co-ops. Or perhaps they got no other applicants.