Do not disturb and Exchange 2010 server

I am using iOS6.0.1 on a 4S. All my contacts are sync'd wirelessly to my company's exchange server.
When using the "do not disturb" feature - how do I create groups? I can see "Everyone" and "No-one" and "Favourites" - but how can I create a group of people that can always contact me?

I can't find how to do this.

Thanks

Mark