Money is a big deal. If you happen to be with others who have similar values then conflicts are low--ie, you all like shopping at costco and having huge
bottles of ketchup in the Refrigerator. On the other hand, if you have a real range of tastes and styles and income things get more complicated. For example if
somebody stays home and cares for domestic tasks and/or does child care for no income and others are making big or small bucks at outside jobs.
Several things that can help are to actively figure out the differences you have regarding money, spending/saving, etc. so they don't come out and bite you from behind. I've had great luck sharing all income/expenses for years with commited partners. Our agreement about income was that everyone had to be doing something that suited them and the family both. What this meant was that if somebody was working at home and the others found it useful or just okay for whatever reasons, then the fact that they were not making income didn't take any benefits away from them. We also didn't want to live with some folks in the family having more money clout than others--like I can buy good clothes and cars while you can't because our society pays child care workers stupidly low and pays atheletes ridiculously high. You get the point. We also put our original savings etc (those we came into the group with) in a pool which we managed together--for investments and major purchases like houses, etc.
All this worked because our basic styles were pretty similar and we really all loved each other. Years later when we added some new folks we found some big differences in their economic styles and it made everything much harder. We needed a whole new system. Some folks have never saved a dime, have gambling problems or make bad investments, don't balance check books, etc.
One thing that always seems to help is to have clear (written is best) agreements so that later nobody can say they were "robbed"--it also helps to keep some books to reflect upon your spending habits if/when issues arise--this can be just the household checkbook if you run most everything through it. Finally it is MOST useful to have some kind of method agreed upon of how you would divide things if you split--if you make this when you love each other and want to take care of each other, then it's easier to stick to when you splitting up and feeling lousy.
Finally there was some info on Kerista's system (Kerista was a commune for about 20 years and had up to 36 people in it--they invented the word polyfidelity and most everyone was in a group marriage with others in the commune) in a recent post. Since I knew and know folks who were part of this, and actually people who did the bookkeeping, I have a different perspective. There were lots of problems with the system and it was by no means simple. Their break up was very traumatic to many and it had to do with distributing money when they split apart. They had made big plans for everything BUT that possibility.