In short, you can queue up by visiting a Battlemaster or entering the zone within 15 minutes of the game start. Only 100 people will be allowed to fight on each side of the battle, to be chosen randomly from the battlemaster and from the zone, with level 80s receiving highest priority.
When Wintergrasp was first introduced, even people who otherwise disliked PvP found it fun, and some old-timers found it be about the closest thing to the fabled world PvP battles of old in Hillsbrad, Ashenvale, and other such places.
Unfortunately, Wintergrasp has found itself the victim of far too much popularity. The fun factor aside, it's also one of the easiest way to grind honor, heirlooms, and epic gear you'll find in game, and it got so overcrowded that it would often destabilize whole servers when a battle was active. Blizzard tried numerous ways to combat these problems, including preventing lowbies from afking in zone to gather free honor and reducing the daily quests to weekly quests.
Apparently, none of these measures have worked to their satisfaction, and instead, they've simply placed a hard cap on the whole zone, arguably turning it into just another battleground that happens to occupy the same space as a PvE farming zone. While Wintergrasp may have been first meant to be a break from battlegrounds or an attempt to give people back the epic World PvP they remembered from the old Hillsbrad days, it seems Blizzard's found the whole concept a bit too overwhelming.
On one hand, it's a bit disappointing to see these changes. It's a lot of fun to get into a good chaotic PvP melee and do some damage. That may be one of the reasons the old hours long version of Alterac Valley was one of my favorites battlegrounds. On the other hand, if it means less lag overall for everyone, it may just be a sacrifice worth making, even if it will suck when one is not chosen for the battle.
Besides, 100 people is still an awful lot, more than, for example, those old 40- person AV days I was waxing nostalgic over a few sentences back. It'll be interesting to see how balance works out on this change, mostly. Will 100 vs. 100 show any glaring flaws on the offense or defense? If nothing else, it'll be a handy excuse to use when you lose, I'm sure.
For your convenience, here's the notes on the system as they appear in the most recent official patch 3.2 PTR notes:
To provide players with a more transparent notification of when Wintergrasp battles occur, as well as better control zone population and stability, several changes have been made.
- Players now have the option to queue for Wintergrasp from a Wintergrasp Battlemaster in any capital city or by simply entering the Wintergrasp zone.
- Queuing will begin 15 minutes before each battle. If chosen, you will automatically be teleported to the zone. Any players in the zone who have not been chosen from the queue will be teleported out when the battle begins.
- The queue system remains active for the entire battle. As soon as a player leaves, a new one will be chosen from the queue.
- Trying to enter Wintergrasp during an active battle for which you have not been chosen will teleport you out. Please note that, as you are now able to fly over Wintergrasp, you will only be teleported out if you try to land and join the battle.
- Level 80 players get higher priority in the queue than lower level players. In addition, a random selection of queued players will be taken from both the Battlemasters and the zone itself.
- The queue will accept up to 100 players from each faction, resulting in a maximum battle of 200 players at a time.
Patch 3.2 will bring about a new 5, 10, and 25 man instance to WoW, and usher in a new 40-man battleground called the Isle of Conquest. WoW.com will have you covered every step of the way, from extensive PTR coverage through the official live release. Check out WoW.com's Guide to Patch 3.2 for all the latest!