Game WISH for this week asks about adding new characters to an existing party. Here's my two cents.
I think Im less in the mainstream of answers on this one. I have always been from the school of grab the character and smash it into the existing party, wiping off the little bits of leftover playdoh that squish out around the edges. This isnt because Im not concerned about party unity or plot smoothing--I actually am, very much so. But I also feel strongly that its not my job as the GM to say, you cant make this character because it doesnt fit into the game/plot structure. On one hand, this is why my Cthulhu party is full of ninjas.... On the other, if a player wants to take on the added difficulties that being a Japanese martial artist touring the world in 1925 is going to incur, Im not going to tell them no. The very fact that these characters are IN the plot means that they are unique among the rabble, and Im not going to really sweat the fact that there werent really a lot of women archaeologists or kung-fu masters running around in Kenya in 1925--there werent really a lot of bloodthirsty cultists or hideous world-devouring monsters there either, so far as I know.
As to the how to introduce them to the party, question, again it seems to me that the chief issue is to get the PLAYER involved, not the character. One follows the other. If we were playing strictly by the rules of logic, no one would ever join an existing Cthulhu party: Ok, youre sitting in the bar, and at the next table you hear a bunch of people discussing the impeding end of the world. They seem to be total lunatics. Oh boy! I pull up my chair to join them. Trying to come up with a logical explanation why ANYONE would want to hitch their star to these nutjobs would drive me crazy; and with the character attrition rate in this game, the issue comes up pretty regularly. My efforts on this are limited to one-sentence summaries of why the new character is interested in the party, and why the party doesnt just shoot them out of hand. Your sister went to boarding school with someone who is now a missionary here in Bombay, and look! There she is on the street! You go over and talk to her, she invites you to lunch, and...whoomp, there it is. The game exists for the players, and bringing the plot (or the character generation process) to a grinding halt every time a new player joins or an old one loses a character seems counterproductive to me. My job is to make sure everyone is having fun and feeling involved, and the haphazard method of introduction accomplishes this in a fast and easy way.