Party Finder isn't available in missions, including lobbies.
Some missions lock after the lobby, so there's just a brief window to join. Party creators have to wait in the lobby to allow randos to join.
It makes sense to add party finder to the menu in lobbies, because ppl starting parties would maybe alternatively want to join another party, instead of waiting for more participants. Maybe someone makes a new party just because they want another difficulty, but the person waiting alone in a lobby wouldn't mind joining and playing that other difficulty, but in the current state they can't see the other party.