The functionality of Groups (cells) needs to be enabled (via control panel) by the Church. Once enabled, it will appear in the app as soon as groups are published. In this icon the member/visitor can access the basic information available and request participation.
The leader (added at the time of group creation) will receive a notification and email informing the request and will be able to allow or deny it.
If you still have questions, check out our playlist of tutorials on Youtube