Multiple companies on one Exchange Srv ?

My question is as follows:

The company I work for is renting out offcie space to multiple companies and they will be leveraging off our existing IT infrastructure. How do I best accommodate these companies on my Exchange Server? Do I create a newsite for each company? I will not be able to buy a whole bunch of PC’s for each company just to install Exchaneg to. So whats the best strategy to tackle this?