My friend and I are also 21 and just got back from Hawaii.

We ended up not renting a car and taking the bus everywhere. But taking the bus isn't a much cheaper option since there are six of you (a monthly bus pass is $27,) and it's very time consuming (2 hours to get to North Shore from Waikiki.)

Enterprise Rent-a-car has a center not far from the airport and does not charge an extra "under 25" fee. You can get rates and make reservations at their site:
http://www.enterprise.com
Extra costs to think of when renting a car though: parking at your hotel will be $7+ a night, gas is approx. anywhere between $1.50 and $1.80 a gallon and if none of you have insurance Enterprise will gladly provide you with some, for a fee (also, there's an extra charge per day for each driver, so if all of you have D.L.s it might not be the best to volunteer that info b/c Enterprise might tack on those charges anyway if you do, even if you aren't all planning on driving.)

Have fun!