So, as indicated by the list of variables that I keep handy for writers, myself included, the first generated romanceable NPC happened to be generated polyamorous. I know very little about polycules, so I seek help from those who might know:
Where is the line between interpersonal psychology and group psychology?
What changes within group psychology when the relationship is a web of sexuality?
How can polycules deal with prospective new members, assuming an equal power dynamic between all existent members?
Supposing that an existent polycule owned a business, how do the group financials work? How does this change when a new member is added?
Ask 6 people with experience of poly what being in one is like and you'll get 12 different answers. It's a huge topic with many variations. There is a lot of resources out there with detailed info, so if you're invested in the subject you'll be able to find plenty of resources. I've never been in a ploy myself, but I know a few people who have (I am active in the London kink scene where poly is relatively common). I'll attempt to answer your questions though:
Where is the line between interpersonal psychology and group psychology?
Depends on the set up. On the one hand, if you have a kitchen table poly, where everything is talked through and agreed amicably then the balance is very even. But if you have a patriarchal poly, with one man at it's head, then the psychology is/can be determined by the male head, so a single lead with a group that follows. To answer that question properly you'd first have to decide exactly what type of poly you were talking about, and then what the role of each individual is to determine their psychology.
What changes within group psychology when the relationship is a web of sexuality?
I'm not sure what you mean by this question. But, again, it depends on the type of poly you have. By definition, every poly has some sort of sexuality (among other things) at it's core, it's kind of the point a lot of the time. To what degree that is a 'web' depends on how open they are with each other.
How can polycules deal with prospective new members, assuming an equal power dynamic between all existent members?
Great question. If everyone is equal, like a kitchen table poly, then I would assume that there are ground rules in place to deal with this. Is it simply two couples who interchange with each other and do not want any more folks involved? Or is it a larger group who are actively recruiting more members? Normally there is some sort of mutual agreement in place about sex and relationships outside of the poly. Again, it depends on the exact type of poly and the viewpoints of those involved.
Supposing that an existent polycule owned a business, how do the group financials work? How does this change when a new member is added?
I've never seen anything like this IRL, so I've no idea how it works in the real world, I'd say it's fairly rare though. If it were me, and I was looking at this for a story, I'd base it on non-poly groups that have financial links, such as a kibbutz or a co-op. Here in London there is a football club that was bought out by the fans, they each hold a tiny share. There are also plenty of cases where locals in a small village have got together to by the village pub or shop when it's faced with closure. I'd research something like these examples to see how group dynamics work with shared finances and then apply my own version of it to the poly I was writing about.
One thing I'll add, most polys I know of collapse under their own weight, they rarely last long term. The longest I've known was a woman who was in a poly for 7 years, but that is rare in my experience (I'm sure that there will be real examples online that prove otherwise though). It's difficult enough managing a relationship with two people, the more you add, the harder it gets.
