There is up to 4 player co-op in AC Unity. You can join together with your friends and complete "Brotherhood" missions. The story missions do not support co-op. Very similar the Watch Dogs is that every plays as Arno but you see differnt re skins of NPCs in your game just like multiplayer in Watch Dogs.

Heist missions are randomized replayable missions you can play for loot. These missions randomly generate loot, guard patterns, and guard positions. These reward stealth by giving more loot to stealthier playthroughs.