[Konstantinos]

You can use a key for enter/exit (if I understood what you wanted to).

pawn Код:

new
    bool: PlayerEntered[ MAX_PLAYERS char ]
;

// OnPlayerConnect:
PlayerEntered{ playerid } = false;

public OnPlayerKeyStateChange( playerid, newkeys, oldkeys )
{
    if( ( newkeys & KEY_SECONDARY_ATTACK ) && !( oldkeys & KEY_SECONDARY_ATTACK ) )
    {
        if( !PlayerEntered{ playerid } )
        {
            // enter..
            PlayerEntered{ playerid } = true;
        }
        else
        {
            // exit..
            PlayerEntered{ playerid } = false;
        }
    }
    return 1;
}