Okay, Phil the guy has already written a very good guide (it got my thumbs up), but while following that guide, I was surprised by the co-op variation of some activities, so I'm writing a guide about the differences between the solo and co-op, just to make strategy easier (Phil if you want to incorporate it in yours, I'll gladly delete mine).
Crowd Control- Can actually get pretty hectic in the later levels, with as many as 5 or 6 stalkers after your celeb, stay on your toes.
Demo Derby- Be quick in later levels or they'll destroy each other before you get the necessary damage in.
Drug Trafficking- No change, easy as pie with a homie to revive you.
Escort- Remember the Ho'ing Diversion? Whoever initiates this activity is in the backseat, pleasuring the client, the partner is driving and avoiding news vans.
Fight Club- The same, only a significant increase in opponents (think it was 8 at level 6).
Fuzz- No change.
Heli Assault- Initiating player is the pilot, partner is on weapons, much easier in co-op.
Insurance Fraud- Higher totals needed, ignore adrenaline zones in favor of interstates (and the lovely fast, non stop big trucks they bring).
Mayhem- Again higher totals needed, fences and Al fresco cafes make this ridiculously simple (my partner and I's highest combo was 214).
Septic Avenger- Initiating player is the only one who can brake the septic truck.
Snatch- You can only pick up one girl at a time each, but you can use separate cars, and go separate places for pickup, insanely easy with a little communication.