Step 1 is to find the group you want to leave. Then you press on "Groups", and then press on the group you want to leave. In this case, "G-2014" under "Sports Club IL" is the group I want to leave.
Step 2 is to find the three dots in the top right corner. When you have pressed on the group you want to leave, you should see the screen shown in the image below.
3. Step is to press "Leave Group", and then you will automatically be removed from this group.
Note that if you have any outstanding payments, you will not be able to leave the last group in the club. The payments must either be paid or waived.