Adding characters to your murder party (part 1)
Imagine that you are running a murder mystery game and it's full - and at the last minute someone wants to bring a friend....
What do you do?
Well, assuming that you don't say "Sorry, we're full", you have several options:
First, you could actually have someone help you host the game - particularly the larger games. You might assign them a particular duty - so that they deal with telegrams, or the horse-racing or whatever the murder mystery involves.
Second, you could give one of the characters an assistant. They would probably have similar goals, but you could change them slightly. Depending on who you gave an assistant, you could also give them a goal to keep an eye on their boss.
I cover this in more detail in part 2.
Third you could create a completely new character. In Death on the Gambia this might be a scientist. One of their goals might be: "You've heard that peanut oil has some remarkable properties. You're an up and coming scientist - find out what you can."
If you have time, you should write out their goals and brief background (you don't need to be as detailed as the "proper" characters). You will also need to tell everyone about the new character as they won't be mentioned on any of the sheets. Finally, you can just print out some of the abilities and items (travel papers, etc) again for them. (I wouldn't give them too much money though as you might unbalance the game.)
Incidentally, Freeform Games have already done this - they have a number of optional characters (most written by their customers) for their games.
Adding even more characters to a game
If you need to add even more characters to your party, then you can do a mixture of the above, but if that still isn't enough you could try one of the following:
First you may have some guests that don't want to be fully involved in the mystery - or perhaps they just want to observe. If that's the case you you could create some generic characters so that they fit in with the party, but don't obstruct the game. If they decide, during the party, that they want to become more involved, you can tell them they have help solve the murder.
For example, in Murder at the Four Deuces
(a 1920 gangsters game), you could have some people
play generic gangsters, cops, politicians and molls to make the club a
bit more crowded. To give them something to do you could set up a
blackjack table, or give them some simple tasks such as
serving drinks and food.
Second you could write a set of completely new characters - with a backstories, their own plots and goals. Be warned, there can be quite a bit of work in doing this - but it has its rewards as well.
If you do take this route, I recommend contacting the publishers of the game that you're adding to. Not to get their permission, but to see if they would be willing to purchase your characters from you once you've completed them. For example, Freeform Games already produce a set of 10 extra characters for their Curse of the Pharaoh game, and I know they are interested in producing more.
The only possible downside is that the publishers will have requirements that you need to meet (they'll have a house-style, if nothing else). Their requirements will probably be more onerous that you would need just for your friends, but as you've got the whole game you should have a pretty good idea of what to expect. So go ahead - write those extra characters!