Wouldn't it make more sense to add a "membership" table?
Individuals would link to individual membership records. Families and Groups would link to a common membership record for their group.
A "membership type" field in the people table could then be used, to insure that individual memberships are truly individual if you find that you need to.
Count ( membership::MembershipID ) defined in the People table would return 1 for individual memberships and more than 1 for group memberships.