I found that the instructions found on LinkedIn via this Google search are all wrong, I'm assuming due to a site redesign since all of those questions were answer.
Try the following instead:
- Log in [of course]
- Click on Groups along the top row:
- Click the name of the group you want to leave.
- Click More... below the group title
- Click "My Settings" in the drop-down
- Click "Leave Group" at the bottom of the page
It works, but is about as byzantine as possible. They even have the leave group button "greyed out" to discourage me from clicking on it.