yes dj bike can hav gears, get one of those with both front and back break and gears.
Check out http://www.pinkbike.com/buysell/
The best bikes to look for in your price range would be Specialized P. Series, Kona DJ bikes, or Giant STPs.
One of the easiest ways to make a bike diverse is the tire setup. Have some low profile tires for riding around town, and some knobbies for the trails.
As long as you have the basic skills of mountain biking down, you'll be able to keep up with your friends on the trails.