I think the term you're looking for is... "common knowledge"[0].
And indeed, there are peculiar things that happen once you cross the thresholds between knowing something, knowing that others know it too, and knowing that everyone knows everyone knows it.
Related to this, and I'm guessing where you were going with your wedding analogy, is the concept of intersubjectivity[1].
Intersubjectivity happens when you have people sharing subjective states or beliefs, and are aware they're sharing it. Pretty much all critical social infrastructure - all of modern life - depends on it. For example:
- Money is an intersubjective phenomenon. It holds value only because - and only for as long as - everyone knows that everyone else in the community will accept it as a means of payment. This shared belief is usually bootstrapped and maintained by a government declaring they'll accept a given form of money for purposes of taxation - but on smaller scales, can also start in many other ways.
- Organizations like governments and corporations are an intersubjective phenomenon. To borrow an example from Harari[2], a corporation like Porsche doesn't really exist in physical space - it's an abstract entity that exists only because, and for as long as, people believe it exists. It's independent of people employed in or running it - the board could fire all employees, and then the government could forcefully replace the board, and Porsche would still be there, with all the factories and offices and salons it owns.
- Companies are bootstrapped into being by performing certain rituals - offering monetary sacrifices to the government and getting the right priests sign the right legal paperwork. These rituals - and all laws in general - are also intersubjective. They have power only because, and for as long as, people believe they do (and believe everyone else believes it too).
It's kind of mind-blowing if you think about it - all the structure of our civilization is just an enormous pile of interconnected shared delusions. But it works, because intersubjective phenomena are extremely resilient - people can stop believing in them individually, but it doesn't matter for as long as enough other people keep the faith. It takes a critical mass of people to lose confidence in a visible way, or a memetic weapon[3], to shut down such an intersubjective phenomenon.
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[0] - https://en.wikipedia.org/wiki/Common_knowledge_(logic)
[1] - https://en.wikipedia.org/wiki/Intersubjectivity
[2] - As much as I remember it. I was introduced to the name "intersubjectivity" through his book, Sapiens.
[3] - People don't have to actually lose faith in something - they just need to believe that enough other people believe that enough other people don't believe anymore.