In our experience - on the cruise in which we turned platinum, the upper right corner of our boarding pass had "Platinum" on it, so we were able to use that Platinum/Diamond line.
In regard to your other question, we frequently travel with people who are not Platinum, and we have either had to join the regular line with them, or meet them on the ship. I have seen people successfully argue with staff until they finally get a person (or two) into the platinum line with them, but I think that is against their policy, so I don't try. Unless they are in the same room with you - then it's fine.
