Leaving a group on the isle can be done in a few different ways. The first way is to go to your group page and click on the Leave Group button at the top of the page. This will remove you from the group immediately.
The second way to leave a group is to go to the settings page for that group. On this page, there will be a section labeled Leave Group. Clicking on this will take you through a confirmation process, after which you will be removed from the group.
The third way to leave a group is to simply stop participating in it. If you no longer post or comment in a group, and don't visit its page, eventually you will be automatically removed from the group by the system.