Free Companies are player made guilds.
Grand Companies are the organizations linked to each city state.

You can already buy a house in any housing zone regardless of which Grand Company you belong too. You are only limited in that you can only own one personal house, one apartment, and one Free Company house per character. You can already be a member of a Free Company associated with any Grand Company even if you belong to a different Grand Company. You are only limited to being a member of one Free Company at a time.

Grand Company restrictions seem to only exist for flavour at this point by granting you access to city specific glamours, bardings, PVP warhorse, ect.