Let me explain my problem... (Roommate Accounting)

Okay, I think the best way to do this is to have me explain what I'm trying to do.

Next year I'm living with 5 others, and I want a sheet to calculate our living expenses, and to figure out how much we owe each other. I have most of the sheet done, each row is an expense, where we can put in 'Grocery Bill' for example, put in how much it is, who paid for it, and the parties involved in the purchase. To the right of that I have columns for how much each person owes for that row. My problem:

I need a way to take the amount of each purchase, break it up by the number of names in the 'parties involved' field, and fill in the appropriote amount for each person's column.

So for example, we spend $100 at the grocery store, the bill is paid for by Travis, and the parties involved in the purchase are Travis, Leo, and Dan. The sheet must recognize that the bill must be split 3 ways, and then know to add $33.33 to Travis', Leo's, and Dan's column for the amount owed.

I've attached the XLS to make this a little bit more easy to understand. Feel free to edit it and repost. I'm also more than happy to post a final version of the file, as I'm sure I'm not the first person to want a sensable way of handling roommate accounting.

Thanks in advance!