Billmonk has the hability to make groups. For instance trips, groups of friends. This features is good to rapidly add friends, and also to add the hability to suggest who pays next. The shuffle feature is also great in groups, in order to one only make a payment to settle debts in a group.

that is certainly one feature which i have on my todo list i'm not quite sure how i'll implement it (hopefully a bit better than billmonk ) - although i'm not sure if/how i'll implement debt shuffling, since a few might think it's a bit annoying.

i think i would rather implement some kind of "event group" or context which users can join. for example when on a trip you create a event group for that and afterward only debts for that trip are shuffled. this way old debts aren't affected, while still keeping the total amount of debts as low as possible.

We have just added a very basic group system - We decided to go with "Context Groups" (at least for now). It doesn't yet allow you to add everyone within the group to a transaction, but rather allows you to categorize transactions and gives you statistics about them. But you can already add (invite) people into your context groups and the next thing will certainly be that you can also add all those contacts of a group into a transaction.

http://tabsplit.net/blog/2012/keeping-track-of-expenses-during-a-trip-with-friends-context-groups/