There are three ways to remove a person from a group. The first is to click on your people tab and then select the person you wish to remove from the group.
Once inside of the profile simple click the Remove option from whatever group you wish to remove them from!
The second way to remove a person from a specific group is to click on the groups section and then click the group you wish to remove people from.
From there click on the Members tab and then hit the remove option for each person that you wish to have removed from that specific group.
You also have the option to remove several from a group at a time. Use the check mark boxes to the left of the person's name and then select Remove From Group button at the top.
The third way is to use the Manage People tab to perform bulk actions.