Co-ops require everyone to donate cash, gold, and Collection Item, to play together.
There is different tabs on the Co-op area. The first tab is called Overview. The shows you the balance of the co-op account (farm dollars and gold), the name, the description and news.
The second tab is called Members. This shows you the members of your co-op.
The third tab is called Chat and Activity. Here is were you can chat with other co-op members and see if they are online.
The fourth tab is called Co-op project. Here is were you can see all your projects. You need to complete these to level up on your co-op.
The final tab is called Research. This is were you have to find ways of doing things.