In my opinion You will struggle to play GRAW co-op online without first finding people to play with.
There are few people online at best of times and 99% of them will not be playing co-op.
IMO the game is good and i`m getting used to the play a bit now but it`s NOT GR and never will be. it causes sooo much lag that it`s very frustrating sometimes and people are always calling cheater because they think they are shot by other people who seem to be still moving and stuff.
You can check who is online here
http://cert-clan.com/graw.php and read the terrible truth that GRIN have not managed what they wanted to do.
The game has HUGE potential and i`ll be playing it to the bitter end but i feel that unless GRIN sort it out the end will be within a year at most.
To sum it up..Get the game for SP as has been said already but unless you already know some co-op players then hang on to your money for a while..You could always google "co-op graw clans" to try and find someone.