6 people total are needed for this trophy. You and 5 of your friends. Not only must they be on your friends list, but they must also be showing up on your friends list in game. If you add people just shortly before attempting this, reboot your PS3 after you add them
, before starting the game. A system reboot helps refresh the in-game friends list with new friends recently added.
Similar to Grand Theft Auto 4, this game uses an in-game friend system that tries
to sync with your PS3 friends list, but starts working incorrectly once you have more than about 50 friends on your PSN list.
If some of your friends aren't showing up on your in-game friend list, then you need to start deleting people who are showing up on your in-game list who don't have the game. You'll probably want to let them know in advance, apologize, and re-add them after this trophy.
Once you can see all 5 of your friends in the in-game friends list, press Start, click "Online" to set up an online game, click "Standard match", "Cruise" and "Custom". It may not be in that order exactly (please correct me if I gave the wrong order or words), but you need to create a private (in this case private is "custom") Cruise. Once you've done that, it'll ask you how many slots, set those to 6 (the servers have been frail, so 6 seems to work best) and set private slots to "all" by pressing left from the default "None". Once you have all private slots, go to your in-game friends list (start, community, friends list) and invite your friends one by one (don't do it too fast as it might not send; the servers have to register each invite sent).
Your friends must know that they will not get notified via PSN, so it helps to have a Text Chat set up for this one, or some kind of group communication like Mumble, Ventrillo, or Skype on PC; or even TrueTrophies chat.
Once the 5th friend joins your game, you will get the trophy. If they also need it, make sure anyone who needs it adds everyone to their friends list so that you all get it in one shot.
Feel free to offer comments and improvements, and I'll even credit you for any changes made to the solution thanks to you.