Saints row 2 and Saints row 3 co-op is a seamless integration of the single player game... Friends online can drop in and drop out at any point in the game, and you always have the choice to allow or disallow it. The only thing that keeps their co-op from being perfect, is the lack of a filter to keep people who are using cheats from joining your game, or activitating cheats within it. However when you are playing with your friends, or just cool people, there is no negative impact on your single player experience at all... there is no real downside to it. They join your game, run around with you for a while, you can choose to bring them along on the next mission, or you can choose to just keep anyoen else from joining.
This is how I want to see it implemented in Cyberpunk 2077. Seamless drop-in and drop-out, open world free roam co-operative play.