GM: You walk through the door into a room. It's maybe 10' by 20'. Bookshelves line two of the walls, with many strange volumes upon them that you don't recognize. Weird statuettes or figurines adorn the table in the center, around which sit four men.
N00b: What do the men look like?
GM: They all wear glasses and T-shirts bearing nerd-related sayings or images. They are in their late 30s or 40s; it's hard to tell because they're kind of doughy. On the table in front of them is a large vinyl mat, some erasable markers, and bottles of water or Code Red Mountain Dew. Each is rummaging through a backpack or satchel and removing papers, books, and some oddly-shaped objects that you recognize as dice. What do you do, hotshot? What DO YOU DO?!
We've all been there, right? The new guy entering an established group, or even the established group receiving a newcomer. It's an uncomfortable situation for either side, with the numbers unevenly matched against the N00b. What's the next step?
1) Set the expectations. Hold up there, hoss! Before the N00b even gets to the game, lay out some ground rules. Explain how often your group meets and where, what kinds of games you play and what style you enjoy. Does everyone bring their own meal, or does the group chip in? If you play every two weeks without fail, make that plain so you don't wind up with a flaky player.
N00b Tip: If the group doesn't volunteer this information, ask. If you don't like the answers, you might want to rethink this whole thing, even if this is the only game in town. Is a bad game group better than no group at all?
2) Agree on a trial period. We're talking about social interaction here, and people can be uncomfortable giving bad news or rejecting someone. As part of the invitation, arrange for the N00b to join you "just to give it a try." Then play together for one or two sessions before extending an official invitation to become a permanent player--or before you say, "We'll let you know." If there's anything hinky, this is the time to shut it down.
N00b Tip: Be cool about this. These players are deciding whether you'll fit into their little family, perhaps one that's been closed to outsiders for a long time. Besides, you want to be able to say, at the end of the trial period, "I'll let you know." If there's anything hinky, this is your chance to dodge a bullet.
3) Start at a starting place. If at all possible, don't be so eager to recruit new players that you bring them into a long-running or almost-over campaign. Suppose you bring a N00b into contact with your favorite characters--and she kills them? Better to run a one-shot adventure, a new system, or a side quest that can be easily retconned if things don't work out.
N00b Tip: Offer to run a game, if you feel comfortable doing that. You won't mess up an existing campaign, and you'll give the group a chance to play all together as PCs--something they couldn't do until now. They'll love you for it, unless you suck. No pressure.
4) Play it cool. In that first session, try not to overwhelm the new player. Chat with them to get them comfortable, then let them find their feet slowly in the game. If they're new to the system or to RPGs in general, answer questions without going into excruciating detail. Cut down on inside jokes for a while.
N00b Tip: Walk a fine line between doing everything and doing nothing. In other words, try to show off your best play, but not at the expense of the other players. Don't argue rules too much. If the new group plays a rule you don't get, or in a way you don't agree with, ask about it without being belligerent. If you don't like the rules they play, make a note of it and decide if you can live with it or not. If not, maybe this ain't your group.
5) Be honest. This is the hardest part of the whole process, and it's also the most important. Once the N00b has gone home, sound out the group. See what they thought. Will this guy fit in? If not, why not? Are the obstacles insurmountable? Did anything in his play style go completely against the grain? If the answer is yes, then the N00b has to be informed of the decision. Yes, it's uncomfortable and may cause hard feelings, but that's better than having a player bollix up an entire game group just because you couldn't break the news.
N00b Tip: Same. After the game, review it in your mind. You don't have to immediately like everyone in the group, but was there anyone or anything that you actively disliked? Were the jokes not to your taste? Was the GM a dick? If so, does he always run the games, or just that one? If the answer is yes, let the group know that you won't be coming back. You needn't say why.
A good gamer is hard to find. I think Diogenes said that, but it's also true that a good group is even harder to come by. Spend the time looking--it's worth it.