To leave a group click the group settings icon.
Click Leave the group. You'll have to confirm that you want to leave the group.
You may eventually have to refresh the browser for the sidebar to update and see that the group is no longer present.
Note: Once you have left a group you can't find and access it again. In case you left the wrong group or need access again, you'll have to ask one of the other team members to re-invite you to the group.