[RyDeR`]

pawn Код:

new
        xString[256]
;
for(new i; i != GetMaxPlayers(); i++)
{
        if(!IsPlayerConnected(i) || IsPlayerNPC(i)) continue;
        new
                pName[20]
        ;
        GetPlayerName(i, pName, 20);
        format(xString, sizeof(xString), "%s%s\n", xString, pName));
}
ShowPlayerDialog(playerid, dialogid, DIALOG_STYLE_LIST, "Online Players:", xString, "Ok", "Exit");

Should work.