Give external users access to resources?

Hello TechNet

I need to know if it's possible to give external exchange users access to a meeting room or a resource in Exchange.

We have Exchange 2007 and 2013, the users in the Exchange 2013 enviroment will need access to resources in the Exchange 2007 enviroment. The most important thing is that users can be able to book meeting rooms and get an instant reply from the meeting room. 

I've had a look at GalSync (http://www.galsync.com/)  but this tool will only synchronize the objects.. so i might as well create a simple powershell script instead to accomplish this.

If needed I will upgrade the Exchange 2007 to 2013, this needs to be done anyway. 

Thanks :) 

Best Regards Bjørn Olav Vangen Aure