If you are a manager of a group, you can easily remove or change group members.

1. Go to your groups at groups.google.com
2. Click My groups (either on the top left or in the center)
3. The next screen lists the groups you belong to;  click the name of the group to see it's settings.
4. Under People click Members
5. Put a check in the box next to the name you want to remove.
6. Click the Remove Member icon
   

If you select many people to remove, ensure that you do not remove yourself.

Learn more about Google Groups.