Everyone stays at their 'normal' initiative. When its your turn, you perform the aid another action, the person you aid gets +2 on their next roll if you succeed. They add the bonus on the grapple check which occurs on their next turn.
If you don't want to have a player meta game whether his friends succeed in helping him or not, you can have the helpers use the action on their turn, but wait to roll when the grappler goes, but the helpers turn has already come and gone when the grappler goes.
Ok, initial grapple check, solved! Thanks!
What about maintaining the grapple? I guess the helpers may use the Aid Another action to help here as well, right?
The helpers don't seem able to assist against breaking the grapple though... it seems that when you try to break the grapple it doesn't matter if you're being assailed by one or half a dozen adversaries, you still have the same chance to make it!
Read the section on Multiple Creatures under the Grapple Action.
"Multiple creatures can also assist another creature in breaking free from a grapple, with each creature that assists (using the Aid Another action) granting a +2 bonus on the grappled creature’s combat maneuver check. (PFRPG 201)"
Also, don't forget that a grappled creature can attempt to free itself using the escape artist skill, which can also benefit from aid another.
I now I see what you're asking about. You're right the rules don't specifically mention that if you have multiple attackers grappling you, that when you attempt to break free you only need to beat the primary attacker's CMD.
Based on a reading of the Aid Another action, I would say that is by design. With Aid Another you can give a +2 bonus to attack OR a +2 bonus to AC (remember that bonuses to AC also help your CMD), you don't get a bonus to both. I would rule that when an ally aids a grapple that the bonus is either +2 to CMB OR +2 to CMD. Essentially you either help them grapple the opponent or you help them to prevent the opponent escaping.
Ths gives me a neat idea for an encounter with swarming morlocks or ratlings.
Indeed. Sufficient CR 1/2 monsters can legitimately threaten even a moderately leveled party due to things like this. For higher levels, you'll have to get more tactical, but the threat of a 7-goblin grapple, pin, tie-up, and coup-de-grace is still slight but real, especially if several more goblins swarm forward to prevent other party members from aiding.