Outpost Attacks(OA) occur about once everyday outside the base (or outpost) you are in. Whenever it says OUTPOST IS UNDER ATTACK it's a best idea to participate and fight the zombies around your base. The main reason why OA exist is to give you exp to level up quicker.
Once the OA is over you'll get EXP REWARD. It depends on how active you were and how many zombies you killed during the OA.

If you fight with other people or the guides, police officers or soldiers, you won't gain alot of EXP(maybe 50% EXP of the zombie if shot by other survivors). It's best to fight at the corner of the area which can avoid other people.
NOTICE: Never ever go to other areas during OA, because the exp you gain in the other area won't count into the EXP REWARD.