You have to unlock and restore the Co-op Center (available at level 19) before you can create your own co-op or join an existing one. You’ll also need 1,000 coins if you want to create a co-op.

Here's how to create a co-op:
1. click the Co-op Center and open the Create co-op tab.
2. Type in a name for your co-op. Choose wisely—you won't be able to change it. Then select a unique badge with a symbol or a flag and a co-op type (these parameters can be changed later):
- Open—other players don't need your permission to join your co-op.
- Closed—only players you invite can join your co-op.
- By request—players request to join your co-op, and you can accept or decline them.
3. click the Create button to finish the process.

There's also a number of ways to join an existing co-op:
1. Choose one from the list of co-ops in the Co-op Center's Search tab. Select the one you like and click Join.
2. Search for a co-op name in the Co-op Center's Search tab.
3. Accept a friend's invitation in the Co-op Center's Inbox tab.
4. Visit a friend who is already in a co-op and learn more about it by clicking the Co-op Center in their town or by clicking the co-op badge next to their name at the top of the screen.