you guys are doing it the hard way, just go join a public free roam and send an invite to all available people and wait untill most of them have joined your posse and go to the select menu and simply join another free roam session. when you do so your possemembers will go with you. send invetations to all the available people you find in the next roam session and when that's done repeat the process untill you reached the maximum and the trophy is unlocked.
that's how i did it and i can assure you it's more easy then struggling with the different timezones we all live in ^^;
For those that still need this trophy, i would try this