Shared calendar with edit rights to external users

I work in a shared building and I would like to create a calendar in our organisation to control meeting room bookings but I would also like to allow other companies in the building to share this calendar and edit items / make bookings. I cant see how I can share an outlook calendar with edit rights - is this possible? so the only other option I can see is to create a sharepoint site with a calendar embedded and allow external users to view / edit this. Any comments or suggestions welcome.