There currently isn’t a built-in way to have one membership instance shared by a household or multiple people.

Depending on how many shared memberships you have, you could create an organization for each household (or other group) and apply the membership to the organization. (http://nationbuilder.com/organization_profile_attributes)

If you have upgraded to the new version of NationBuilder, you could also accomplish a similar functionality by replacing memberships with paths, but that takes a lot more upkeep and can get real messy real fast.