For those who are unfamiliar with the SR3 Co-Op system, you don't need to start at the same time, you don't need to stay for the entire adventure; it could just be one mission or just a crazy hang out time in Steelport.

When you go to start a campaign, click Co-Op instead of Campaign if you want to host. There is only the host and one guest, but you can host different guests and don't need to have started as co-op to be a guest. you can decide who can offer to join and you can choose to start out as just a single player campaign instead until you find some fellow players. It's the same as the normal singleplayer experience.

Hope to see y'all there for some shinadigans in Steelport.

btw here is an in depth list of what will happen if you Co-Op [for players who have already played and wonder about their stats/assets]

http://gaming.stackexchange.com/questions/37566/when-playing-saints-row-3-in-co-op-does-my-progress-carry-over-to-my-single-pla