If someone no longer needs to be a member of a certain group (because they have moved to a different department, for instance), you can edit the member list that person is in by taking these four steps.
Go to the group with the relevant member list and choose the following:
What you can do
1. Change the name of the member list
2. Adjust the permissions for those on the list
3. Add members to the list
4. Delete members from the list