Managing your group memberships is done through the "My groups" tab. To find this tab, simply click on the Groups tab at the top of any page, then look below the breadcrumb: there are three tabs--"Groups," "My groups," and "Group activity."
Click on the "My Groups" tab, and you should see an alphabetical list of your groups, with a link to the group's "Manager" (the administrator) and an Edit membership link.
Click on the link, and and you are taken to the Manage membership page, with a Remove membership from this group link.
Click on the link, and you should receive a message asking "Are you sure you want to remove [username] from the group [group name] forum?", along with a Remove button and a Cancel link. Click on the Remove button, and you have left the group.
If you are the administrator of a group, leaving is a bit more difficult, as a group is required to always have at least one administrator. To grant administrator privileges to a group member, see Administering Groups.
