It's very simple to quit a group. First, log in to groupvalet.com and click the "My Groups" tab. Find the group you want to leave and click on "Go to Group Homepage". Once there, in the left sidebar click "Quit Group" and confirm with "Yes - Leave this group". You will stop receiving news about the group and you will not be able to sign up for any of its activities. This cannot be done in the app.
Note - if you are the Group Captain, you cannot quit your own group. Instead, you can mark the group Not Active or change the captain, both from Group Settings.