[Threshold]

https://sampwiki.blast.hk/wiki/SetPlayerVirtualWorld

He was probably using this function, so it was a different virtual world for every player.
Ways to do this are:

pawn Code:

CMD:enter(playerid, params[])
{
    if(IsPlayerInRangeOfPoint(...
    {
        SetPlayerPos(playerid, ....
        SetPlayerVirtualWorld(playerid, playerid); //Why is there 2 playerid\'s here? Because nobody has the same player id, so there will never be two players in the same virtual world at once.
    }
    return 1;
}

Or without the use of playerid and house id instead:

pawn Code:

CMD:enter(playerid, params[])
{
    if(IsPlayerInRangeOfPoint(...
    {
        SetPlayerPos(playerid, ....
        SetPlayerVirtualWorld(playerid, HouseInfo[houseidhere][ID]); //Nobody has the same house ID, unless they\'re both entering the same house.
    }
    return 1;
}

The second method is a better option, especially in the cases when you have it set so other players can enter your house.