How does multiplayer work in PoE2?

Hello I am a newcomer to path of exile series. I preordered new game and already excited. I see there is a multiplayer in this game. So how does it exactly work? Is it like we see other players in the world going around like any other mmo? Or we invite people to our game world and play together? How does dungeons and story effected by multiplayer? My coworker also bought the game and we are wondering if we can play together. Lets say I am in act 2 and my friend is in act 1 in story mode. Can we still play together? A lot of confussion on my side.