Over at the World of Warcraft forums, gamer Smellyhead is reporting a bug instance whenever he tries to join the Alterac Valley battlefield. Apparently, an error message prompt appears and says that the group is too full.

Blizzard bluepost Hortus gave an immediate reply, squashing any thought that it is a bug. Straight from the community manager: "You can only join AV as a 5 person party. Which is why the button says 'Join as party' instead of 'Join as Group' as it does for other battlegrounds."

The Alterac Valley is home to one of the fiercest WoW battles between the Horde and the Alliance which is why updates are always done to ensure parity. Naturally, you will only be fighting a losing cause when you head there alone