[X337]

Код:

new Iterator:Gangs[4]<MAX_PLAYERS>;

Iter_Init(Gangs); // You need to use this for multidimensional array
Iter_Add(Gangs[2], playerid);

foreach(new i : Gangs[2])
{
	print("Player id %d is a member of gang id 2", i);
}

You should use Iter_Init for multidimensional array.
And i guess you're trying to insert playerid into gang iterator, so i've changed the way you declare "Gangs" variable and the loop code.