Arathi Basin (AB) is a 15v15 node-based battleground.  You can join as a group.  If you are outnumbered, it does not matter, as the game will not attempt to balance numbers by preventing the opposition faction from joining.  A minimum of 8 players is required for a battleground to continue, otherwise it will automatically end.  However, because of the rate of points gain for 5 capping, you should be able to finish before your time expires.

Aim

The aim of AB is to reach 2000 resources before the opposition.  You achieve this by capturing nodes, which look like a flag.  The more nodes your team controls, the faster your resources will increase.  The table below represents this.

You also gain reputation with the Arathi Basin faction for your side (The Defilers for Horde or The League of Arathor for Alliance players).  Increasing your faction with these groups unlocks access to some excellent items.  Each 200 resources gives you 10 reputation with your respective faction, up to a total of 100 for a victory.  On AB weekends, you get 10 reputation per 150 resources, which is 130 per win.  You can also get 800 rep from the quest The Battle for Arathi Basin!, which requires you to assault 4 bases.  You do not have to click the flag yourself, being in the raid and near the flag when it is capture will result in successful completion of the quest.