You can leave a group that you are a member of by clicking on “Settings” in the left side menu on the group's homepage.
A red "Leave Group" button will appear beneath the group name at the top of the screen. Click that button in order to leave the group.
Note: This applies to Action Teams, Custom Groups, and role-based groups (Group Leaders, Liaisons, etc.) If you need to change your CCL chapter, see Changing Your CCL Chapter Membership.