More thoughts.
The issue would be whether the co-op had the legal right to make the copyright transfer.
There needs to be an explicit layout of the different hats that everyone is wearing, one legal relationship for the co-op member as “employee-of-the-co-operative”, and the other legal relationship for the co-op member as “joint-owner-of-the-co-operative”.
Making those different legal relationships explicitly clear would go a long way towards clearing up any legal issues that may arise.