[Codeah]

Код:

new string[128], pName[MAX_PLAYER_NAME]; // 1
GetPlayerName(playerid,pName,MAX_PLAYER_NAME);
format(string,sizeof(string)," %s has joined the server",pName); // 2
SendClientMessageToAll(-1,string);

1. String is way too big (I recommend 24 since an username can only be 24 chars long.)
2. Missed end quote and sizeof string = sizeof(string)