It is 100% Co-op. They can mess you over, but they are only hurting themselves in the short and long run.
It's only 100% cooperative if all members playing are cooperating 100% of the time.
Since during the main story you can fight each other in the arena, steal from each other, and can sometimes earn experience independent of the other members, it's not all cooperative.
Since you join with other players who have different goals and desires than you, it's not 100% cooperative.
If it were 100% cooperative OP wouldn't need to ask for a way to force fair loot distribution.
