There's a lot that shows in the preview - unfortunately a lot of players are either still not familiar with all the highlights in the preview (Restrictions, mic on/off, etc) or are not looking hard enough. We once had a guy in a Corvette Race car join a room that was specifically slated as 'Street'. He left after he was informed of his error. As one of the members here stated: If they find out it's not what they came for rarely will they change the car they
want to race.
There may be also some confusion in the minds of more casual players ('Hey! It's a Chevy Corvette! It's a Street car!') Hosting can be hard work; this is where there should be that constant flow of info between Host and Players.
I agree. Nationality doesn't matter. In any case what really does the location flag mean? More often than not just the location of the player, not what he eats or looks like or whether they sleep with the Kama Sutra under their pillow. I've played with dozens of nationalities.
We shouldn't ignore some facts, though:
People can be racist. They can be hyper-sexual, scatological, silent, 'strange' - or 'racing maniacs' because the particular culture they were bred in influenced the imprinting of these characteristics. These are labels we give people so that we can make some sense of the disorder around us. And of course we have to constantly fight the notion of archetypes, stereotypes, and other types of bigotry and mental programming.
This is why we naturally gravitate towards cultures that are somewhat alike - I see very successful rooms of USA/CANADA/UK combinations that I have raced in for hours - but we've had the occasional driver from Finland, Portugal, even Tunisia, who melded successfully into the group, raced and left. There are many times I've seen the German flag pop up in a USA/Canada room, and it's usually is someone polite. And a good racer!
We have to understand as Hosts, that if someone cares to join a North American Lobby (after possibly looking at his home base first) from Europe or other 'further-away'- countries, then they must be racers looking to match their skills against international players. I've found many of these 'roamers' to be great racers.

Did they leave right away or did you bounce them?
As I mentioned before - starting a race is crucial and should not be taken lightly. Either have it on Auto-cylce or make sure everybody knows. At least a two-minute Text warning - for players who enter the Lobby just when you're ready to drop the flag.
And never ever give in to people who demand you start a race in a room titled 'Free Run'. People take 'Free Run to be a mode they can enjoy their cars in modes
other than racing.
The new update fixes some of these problems.
Mics on or mics off? That is a whole topic in itself.

Someone take a poll . . . .
I mentioned a player who plays 'Race Director'; he's on mic and uses it to marshal the players. He's very polite, doesn't talk unless absolutely necessary, and most of the times I've raced in his room, it's only one or two others that are miked (I was - so I was chatting with him now and then - questions about limits, cars, etc, and also to confirm that I was ready to race or sitting it out.)
The rest of the players answered through text. It worked out.
Rooms can be either 'Public' (which means any and everybody can drop in) - or 'Friends' (which of course means one's PSN friends.)
If it's 'Friends' - okay, no problem. You're doing your thing, it can be anything, you're among 'friends' - if everybody is miked, then you're good to go for just about anything. The Title of the Room can be anything you want - since it's actually a 'code' that let's you know that that is the Lobby. Also would show when you check the 'Show Friend's Rooms' Box.
As soon as a Lobby is Public it doesn't matter what is on the Title Bar - people will stick their noses in. It's an Open Room. If the Room was made by a bunch of people who were not on each other's Friends lists, and yet wanted to congregate in a Public Room, there will be strangers dropping in . . . . and
kicked? It's tough to try and run a Room like that. The only way that kind of Lobby succeeds is if the host manages to synch the new 'strangers' in, and they become part of the regular pack that visits that particular Lobby all the time.
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Here's a few more tips that came to mind:
If you're running a Track day, say at the Nurb or Silverstone, study the Track Day Rules (widely available on the Internet) and enforce them. Many drivers use this as a simulator - a 'role-playing' game, if you wish, that permits them to practise Track Day behaviour before they go ahead and do the real thing.
Free Run?
Specify the rules and make it known to newcomers if they are confused. Where can they park? Can they run in reverse? Is there a Convoy? Etc.....
I agree - reverse grid shouldn't really bother a faster driver - there is more of a challenge, and more satisfaction to be achieved in cleanly overtaking the slower drivers and keeping out of their way when they spin out. As well - faster drivers are usually better drivers - and know where the brake pedal is. I would rather have an experienced driver chase me, than an amateur. You won't believe the amount of times I've been rammed by some tenderfoot behind me with a lead foot and no concept of 'corner-entry' speed. Experienced drivers chase me down, stalk me, and pass me cleanly when they can. Because they
can.
Gran Turismo is definitely not just about racing. That's at the core - but around that core is wrapped many layers of activity - and Online activity that has nothing to do with pure racing but much ado about everything else Automotive - collecting, detailing, photographing, tuning, driving for pleasure, and of course (hopefully soon to come) trading. We like to share stuff - and socially, online, there is a lot of sharing . . . and such particular Lobbies will be hosted by players that share that view - and there may be no racing taking place at all in the room.
The new update - Ver. 1.05 - has further tweaks for Hosts. Welcome additions, I would think. We Hosts can never have too many filters.