I don't know of any teams who are specifically riding mountain bikes, but I remember from past years that lots of people do.
As far as using a mountain bike goes, you will have to work harder, but as I said, lots of people do it.
There are a few things you can do to make the ride easier.
One: Buy city/commuter tires for your bike. They have a lot less rolling resistance than knobbies. I think most bike shops carry them.
Second, make sure your seat is at the right height for road biking. One trick I learned: Put your heel on the pedal, then your leg should be locked straight at the bottom of the pedal stroke. Now, under normal biking you ride on the balls of your feet, so your leg will have just a slight bend in it.
Lastly, if your bike has rear suspension, consider getting a hard-tail. On the road, a rear spring will absorb a good proportion of your pedal energy, instead of transferring it to the wheels. Some full suspension bikes have the ability to lock the rear spring. Use this feature if you have it.
Hope this helps.
Anyone else have additional pointers.....................?