Well, i don't know much about multiplayer yet, but i've seen two icons so far for it. From what i've heard you can just roam about (you can't go too far away from each other as it isn't split screen ) and i think there are also special missions you can do together (not sure tho...)
Also, i just found a cool thing (kind-of like the taxi at the hospital when you die in a mission), i was doing this mission for tenpenny where you have to drive to the docks before you do much (and i'd tried it 2 times already), and i got on a bike and a d-pad flashed up in the corner saying TRIP SKIP!!!, so i pressed right and then it said "later that day..." and took me to the docks