Sleeping on the floor with your toon. I'm not joking, some of my friend started a PUG that way with a really fun guy from the West Coast in DDO once.
You know, I don't think there is a good "way", it really depends on the person you are trying to speak to.
I used "lfg" features to join group and talked to them afterwards, but I hate, hate, hate blind group invite (people who see you not grouped and send you a group invite without asking first).
I received /tell from people a few times asking if I needed something or want to join their guild in different MMOs. Unless it's spamming or insults, I don't mind it. The point is to be polite about it.
So in other word: be respectful and mindful of who you are talking to.
Community is the crazy notion that massively multiplayer games are more interesting when other players matter. - Community: The 3rd Element blog-