Energy co-ops can take various forms, they can simply be buying groups for the equiment (usually solar PV panels, cables inverters etc.) for installation on members’ own roofs, or they can entail purchasing land for either solar farms or a wind turbine. They could include insulation for your home, to make it more energy-efficient, and the cost can be spread over up to 20-25 years.
Look at some of the examples in the panel bottom right for a better idea of how many different co-ops operate.