Share calendar with other company

Hello,

We are sharing 3 meeting rooms and would like to use Outlook to manage a shared calendar instead of writing the reservations on the door.

We are on Office 365 and they are migrating in a couple of month to Office 365, but are on Exchange 2007 actually.

Is there a way to create calendar on 1 site and give access to the other company?

Thanks for your help.

Stephane