It depends on what you want to do. By default (why has been discussed elsewhere on this site) all single sisters and perspective elders are assigned to the HP group. There is no way to change this. However if the Elders quorum has a route they would like to add a single sister or perspective elder to they can by:
going to the companionship tab and then adding them to the desired companionship.
If they dissolve the companionship or remove the family from the route the family will revert back to the HP unassigned list. There is currently no way for a family to be moved to the unassigned list for the Elders quorum from the HP unassigned list.