[Basssiiie]

Код:

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate == PLAYER_STATE_DRIVER && oldstate != PLAYER_STATE_DRIVER) // Player enters the vehicle as driver
    {
        SendClientMessage(playerid, WHATEVERCOLORHERE, "Welcome in the vehicle, dear sir. :)");
    }
    return 1;
}

Piece of cake.