Problem:
You can’t seem to find a user as a member of a group to remove them from the site
Solution:
- Click on the cog in the upper right
- Click on Library Settings
- Click on Permissions from this document library
- Click on any existing group
- At the top, in the URL, you will see (at the end), MembershipGroupId=??, change the last number to 0
- Select the users and remove them