For the relationship where everyone is with everyone, and the introduction of a new person requires everyone to be into them (and thus each addition becomes exponentially more and more unlikely, as the group gets bigger), I refer to this as a "Pod", where its all one unit of people together.
Whereas when its not everyone connected to everyone, and its more open ended and people come and go, I refer to that as a Polycule (from Molecule), as in a "chain" of connections.
Typically the former simply just never gets very big, because it inherently gets very strained as it grows beyond even 4-5 people at most. Humans just cant sustain that many intimate relationships at once.
Polycules can go infinite though, cause any one specific individual in the "chain" of people can simply just be with only 2 or maybe 3 themselves, but that infinite chaining can just go on and on and on, without any individual even knowing how far it even goes.
I personally am not into polycules, Ive never seen one actually sustain and long term, every single polycule Ive witnessed disintegrates and fractures over time into different groups, it can get petty, people can get hurt, I just personally try to steer clear of such stuff cuz Ive yet to actually see someone long term defy the pattern Ive seen.
And by long term I mean 10+ years.
If the polycule is purely transient and people can just float in and out and everyone involved is cool with that, thats fine, but I consider that less of a polycule at that point and more just a bunch of open relationships. To be a Polycule in my eyes it has to have longevity and be non transient, each individual "link" of the chain is people going steady. A whole buncha people just having transient relationships, friends with benefits, one night stands, etc, thats not a polycule, thats just swinging.
Which is cool and I dont hate on it, but it's just not the same thing and I try to ensure that distinction in lexicon is consistent.