This is already there and has been for a long time. When you click "Complete Group", the group billing form has a box above it that shows each member of the group with their name. To the right is a column labeled "Action" which has two icons... one to EDIT the member and one to DELETE the member from the group.
The only way you would not see a DELETE icon there would be:
1) You've modified the PHP file that constructs this box, or
2) You have a minimum group size set for the event and you're at the minimum size already. You can't delete a member if it would cause your group size to drop below the minimum. Your group would have to be at least one more than the group minimum for delete options to show.
Nathan is no longer affiliated with DTH since the recent acquisition. You can connect with him and get any type of Joomla website help at
www.JoomlaEmployee.com
.