Some things that need doing:

Rename relations:
  post.person needs to be post.owner
  group.people might want to be group.members
  person.owner should probably be person.user

Refactor receive:
  user.receive is a little horrifying right now.
  Maybe make a model.import or model.perform function which can be called on all the classes.
  Definitely move the varied signature verification methods into a unified object.signature_valid?
    or signatures_valid?