You can directly edit deformer set membership with the Edit Membership Tool.
To edit deformer set membership with the Edit Membership Tool
The members of the deformer set whose joint you selected are displayed in yellow. This set is the currently selected set. Members of other sets are displayed in other colors. Points displayed in dark red are not in a set.
The selected points are now displayed in yellow, indicating they are in the currently selected set.
The selected points are now displayed in dark red, indicating they are currently not in a set.