From memory only I think anyone was welcome at any of the Co-op stores which often had slightly lower prices across the range of goods sold. As Radiorails says the Divi was an added incentive for families on a lower income to join.
Being a member was also something of a tradition, Mom was in the Co-op so you followed suit.
There was also, I believe, a bit of social snobbery in that people of a supposedly higher station in life could afford not to be seen shopping there, did they have a Co-Op in areas such as Solihull or Sutton Coalfield.
One of my old school mates went to work as an assistant at a local Co-Op, and I remember that this was considered to be lesser than working in "a normal shop"
Don't think this reflects on my own thoughts as I threw in an office job (leading to a career ?) to work in a hardware shop whilst waiting on National Service call up. When I told my boss what I was doing ( at a higher pay ) he said something to the effect that some people would do anything for money. I believe they went out of business a few years later.
Cheers Tim