The Members section of your Group displays a full list of the group’s members, and their status as an Administrator or a player. The Group Administrator can remove members or make them Administrators as well.
Go to the “Members” tab of your group > click “Remove member”. They will not receive a notification that they have been removed.
Go to the the Members tab of your group > Find the member who you want to ban > Slide their photo/name from left to right > Tap “Ban Member”. They will not receive a notification that they have been removed.