I'm not sure polyamorous is a very helpful term, because it can mean so many things.
It could be a group which all live together. Or only some of them live together. Or none of them. People involved could be heterosexual, gay or bi. They might all be sexually involved with each other, or only with particular individuals in the group. They might go out as a group or only in couples. They might be a closed group, or open.
Behaviours outside of any particular agreement would be cheating, but you can't assume which behaviours that would include for any given group, whereas in a monogamous couple, you can assume anyone outside the two would be cheating (though obviously some couples may agree to open their marriage in one way or another.)
I think everyone involved needs to be really good at communication and lots of people really aren't, even in monogamous relationships. Society is geared up to monogamous couples (and even now, the default is still heterosexual.) The more people involved, the more logistically challenging it can get (as it can be in any busy family.)
I think you do need to let others know in some ways - if I were in such a relationship, I'd want to spend events like birthdays and Christmas with everyone involved, but I'd also want everyone involved to spend time together, even if not everyone involved were sexually involved together. If outsiders are involved, whether invited or doing the invitations, they'd need to know for numbers.
People will mostly only know the poly people who go on about it, because the ones who keep it private... won't mention it to you... so you could know more poly people than you're aware of. Who knows?