The trouble with one link for everyone
A public join link is convenient for exactly one person: the one who forgets they posted it. It lands in a group chat, gets forwarded to a colleague who was never invited, and turns up somewhere you cannot recall. You lose the thread of who is meant to be in the room, which is precisely the trust a closed session is built on.
A private link per registrant fixes the leak at the source. Access is issued to a specific name and email who asked for a seat, so there is no single address to circulate, and the room is made of the people you actually registered.