I would assume the mansion will still be unlocked, as the owner is still expecting you even though you missed the first dinner, so I suppose just go on in and do whatever your character would until either the rest of the guests are up and you bump into someone, or until Rose makes a post for Ian the butler, which would probably include him coming to find and greet you wherever you are at the time. Shouldn't be long either, as Rose's next post will be Ian waking up the guests, so she'll more than likely take care of you too during that.