Language help: expressing unease with someone

Background: My friends are in Tokyo from overseas this month and we're spending a bunch of weekends together while they're here. This weekend, we organized something of a drinking party with local friends whom we met last year and want to hang out with again.

There's sort of a problem, though. While the main organizers of this thing are a couple of the overseas friends and I, one of the Japanese guys wants to invite someone who we didn't explicitly invite. This isn't usually a problem (this seems to happen a lot here? is inviting others of your own accord to a party you did not organize not considered rude..?) but this particular person makes me extremely uncomfortable. I'm not sure the guy who wants to invite him knows this, and I don't want to make any trouble because those two are friends... but what would be the best way to passively express my hesitation about this person being invited?

I mean I am -really- not comfortable having him there to the point that I don't want to go if he does. I know that may sound childish, but there's a good reason and it's not a matter of being a vendetta; it's a matter of my comfort. I did have an incident with this guy before that doesn't need to be known to everyone going, so I just really don't know how to go about this politely and discretely.

Thanks for hanging in there for the long read. For any responders -- my Japanese is fine; ideas of what to say in characters/kanji instead of romaji are preferable.

If he has not been invited: All I can think of is to say できたら その人は 呼ばないで くれたら うれしいな… ("I'd be happy if you could go without inviting that person...") and hope that the guy picks up on your tone. Otherwise you'd have to explain why.
If he doesn't know the situation, he might go back to his friend and ask about it, then if this guy (the one you want to avoid) hears that it came from you, he might not show up.

If that guy has already been invited, and if you could live with that, and if you don't want to create a hassle, you can tell him: 次からは、別の友達を呼びたい時は、その人を呼んでいいかどうか、まず幹事に確認してね ("From next time on, if you want to invite another friend, please check first with the organizers if it's ok to invite that person, OK?") With that, he might pick up the message that he's done something a bit...not right, if not rude. If he doesn't get it, there isn't anything you can do about it.

grendel,

Yes, it certainly is rude to invite a third party without the approval of the host.

But as suggested, isn't he just asking you if he could invite someone? Or has he already invited that someone without the approval of the hosts? If it's the former, that's normal. All the host has to do is to say "No." If it's the latter, that is considered rude in Japan as well.

Either way, all you have to say is that this is going to be a more of a closed party. Perhaps you can say something like you want it to be a party just for those who know each other, or perhaps you can say that there's no room, or perhaps it can be a mixture of both or more. Make it simple so that you won't have to tell a white lie.

Examples:
ごめんなさい。今回は知ってる人同士だけで集まりたい、せっかくだけど、また今度にしてもらってくれますか？
ごめんなさい。定員オーバーになっちゃうんで、またの機会にお願いしたいんだけど。

If he had already invited that someone, that's his problem. All he has to do is to tell that someone that he made a mistake and that the party hosts have decided to have it be a more of a relaxed atmosphere just for people who are familiar with each other.

Personally, I don't think you have to explain that you don't want that someone there, unless you really have to. These things happen. An invitee might think that it's a good opportunity to invite someone else, or might even accidentally think that (s)he can go ahead and invite that person since a lot of the party guests are mutual friends. They usually mean no harm. You'd want to let them down as easy as possible.