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