To add a volunteer to an existing family, go to the Volunteers pane and edit the volunteer you wish to add to the family. On the left side of the profile screen, it will say "None" in the family section - click on "None" as this is a drop-down menu, then select the name of the family this volunteer should be added to. Click the OK button and the volunteer will then be linked to that family.
You can accomplish the same thing by going to the Family Groupings pane on the left side of MSP, finding and editing the family, then use the "Add" button on the upper left side to select the new volunteer who should be added to the family.
