After opening a group's admin page, open the Assign Members Manually tab.
Assign Members Manually
A list of your club members will be displayed; click one of the Add buttons to add them to the group.
You can remove club members previously added to a group by clicking the red Remove button that displays when they're added to the group.
Members that are added manually will always be included in the group list, regardless of whether or not they satisfy the smart group rules.