Firstly, you have to unlock the Co-Op (available at level 7). After that you will be able to create your own Co-Op or join someone else’s. If you want to create your own Co-Op, you should pay special Coins fee.
Guide to create a Co-Op:
1. Tap the Co-Op building and open the Create tab.
2. Type in a name of your Co-Op. Bear in mind that you won't be able to change it. Then type a description that would help you to attract attention of your fellow farmers. After that you can select preferable language and your Co-Op’s symbol. Last things to choose – minimum level of your future members and your Co-Op type (can be changed later):
3. Tap Create to finish the process.
Guide to join an already existing co-op:
1. You can choose one from the list in the Co-op's Search tab. Select the one you find suitable and tap Join.
2. If you want to find something more definite, you can search for a Co-Op name in the Co-Op's Search tab.
3. Accept a friend's invitation in the Co-Op's Inbox tab.