Yeah it has Multiplayer. Its kind of hard to explain, but its similar to GTA in a way to where everyone will be in a lobby (for the city at least). Then when you want to do quests, you go to the quest board and accept one and it will send a message to the lobby that so and so has posted a quest. You will get a prompt on your screen showing your party and you can wait for folks to join, or you can just start your quest. When you start the quest you can still send out whats called an SOS Flare and that will send another message to folks in your lobby that a quest is joinable and people usually jump in at that point to help you out.
4 people (including yourself) can play a quest/mission at one time. You can start an expedition as well which is just a free roam type thing and folks can join you. Expeditions are primarily for exploring and gathering supplies so that you can craft stuff.
The more people you have with you in a quest/expedition, the more difficult monsters become. Its really fun and the monsters look cool as hell. There are so many different types of weapons and armor and upgrades and just little things in the game that Im still learning. Ive put 10 hours into the game so far and barely progressed at all lol.
Its definitely a lot more fun playing with friends or doing Co Op. I'd recommend getting it asap if you can. I got a few IC Folks that I play with on there, you can run with us if we're all on at the same time.