Also multiple opponents may attempt to grapple one opponent. For each additional grappler, add a +1 to attack roll of the main attacker. If the grapple attack is successful, then all grapplers are said to have a hold on the target. Always consider the main attacker the one with the highest level. In the round following a successful grapple, the grappler(s) may attempt to force them prone with a successful attack. The multiple opponents bonus also applies.
A held combatant may attempt to free himself in the next round by rolling a successful attack roll. They suffer a -1 penalty per grappler having a hold. If they are prone, they additionally suffer the normal -4 to the attack. A held combatant may not perform any other combat action or movement than attempt to free himself.
Since size matters in grappling attacks or freeing attempts, you may want to also consider these modifiers:
If an attacker is Small, they have +0 versus other Small, -2 versus Medium, -4 versus Large.
If an attacker is Medium, they have +2 versus Small, +0 versus Medium, -2 versus Large
If an attacker is Large, they have +4 versus Small, +2 versus Medium, +0 versus Large
Remember that any penalty is applied ONCE to a single die roll, and that bonuses are always cumulative.