|Actually, what you are asking for might be pretty hard to build, mainly because of this requirement:|
4) and update amounts that people owe each other
If I spend $4 and want to split it with you, Bob and Sue, everyone owes me $1.
If Bob buys spends $3 and splits it between himself, Sue and me, Sue owes him $1 but him and I are even.
If Sue spends $8 and splits it between herself, you, me and Steve, you and Steve owe her $2, but I only owe her $1.
Of course, you still owe me $1, so why not just give it to Sue for me and I'll be square with everyone?
Do you see how it can get pretty complicated really fast? I'm not sure how you would set that up in a spreadsheet with the ability to keep adding items and constantly updating who owes who what.
In addition, the spreadsheet would also have to account for payments made between the parties.
If I have time to play, I might give this a shot, but nothing jumps right out at me at this time.