According to their Wikipedia (
https://en.wikipedia.org/wiki/REI), REI is a "consumers' co-operative". This is a different thing than a "workers' cooperative". My understanding is that most grocery co-ops are organized as consumers co-ops, or something close to it.
The grocery co-op near me has occasionally faced the prospect of the formation of an employee/worker union. I imagine that unionization is a thing that doesn't happen (or get seriously discussed) in a workers' co-op.
I find this distinction vaguely fascinating, but (apparently) not enough for me to actually get informed about it.