New Player - Established "Static" Group
Should do - If this is a "static gaming group" then discuss with the group the prospect of someone new joining in, gauge their reaction and take on board what they think. If it's a goer then have an ice breaker session giving everyone a chance to get to know each other outside the focus of the gaming table. In addition to that and keeping the "Is everyone having fun? Including you?" principle to the forefront of my thinking everyone needs to give the new player time to learn as they play and encourage them to try things even if that means they are "doing it wrong" (whatever that means).
Shouldn't do - Don't drop the player into a game (RPGs specifically) where their actions are marginalised by the established plot. Also if the player is entirely new to the hobby at large then try to avoid using jargon that the new player won't get.
New Player - Fluid Group
Should do - In the "fluid group" it's much easier to add a new player into the mix. There's less "prep" needed to get everyone on board with it. Also through channelling the "Is everyone having fun? Including you?" principle there is still that strong need to ensure that the new player is indeed having fun.
Shouldn't do - The jargon element above still applies as does the New Game - New Group shouldn't do element of Introducing a New Game.
New Player - New Group
Should do - This is really the blank slate of situations. The group has never existed in any real form before, the members may have connections with others away from the gaming table and yet they may not. The only key should do that I have come across in this situation is to take it slow. Don't try and force the group dynamic and I'd recommend playing some cooperative board games to help the players find their rhythm in how to play together and individually develop their own gaming styles.
Shouldn't do - Avoid games that have direct conflict as a key mechanic, unless of course that conflict is humorous in it's delivery (Gloom). This is for a variety of reasons, one of which is again about that fun thing but also simply down to trying to engender a healthy group dynamic.
Any other New Player thoughts out there?