Whta you're looking for is called the TailGate Club. Information can be found here: Tailgate Club
Welcome to the Redskins TailGate Club
...Washington's biggest and best pre-game TailGate party in a safe and secure pavilion next to the stadium, plus lower bowl seat locations
Starting 3 hours before every pre- and regular-season game, you don't want to miss the fun, the games and the "Hogettes-inspired", All-You-Can-Eat Barbeque! The only thing you'll miss is a long wait to enter the stadium!

The TailGate club passes are sold as a seaon package, but you can easily find the special wristband you need to enter on DC Craigslist site. Wristbands are going for about $35 each