Click on the People tab on the left side of the screen. Then, select the person you wish to remove by clicking on their name.
From there, you will see a quick overview of their information. You can remove them from a specific group by clicking the “X” icon to the right of the group name.
You can also remove them from a group within the Members tab of a group. Open the group and view the Members Tab. On the right side of a person's name, you can click the trash can icon to remove them from that group.