A heads up for ye' beginners
PvP is not like the other games by FromSoft, it's usually much faster paced.
There is a password system in place for private summoning. In the Network options at the in-game menus.
You must have, and then use an Insight to summon another Hunter for cooperation. Insight is used when you summon another player or NPC, and only when they are summoned.
Be warned, invaders will start to be summoned when you initiate cooperation by ringing the Beckoning Bell.
Invaders tend be be very aggressive, and will show you no mercy. There are a few exceptions to this.