Has this ever happened to you? You receive a call for help, and when you try to answer it, you get a notice says either : "Can not port, dungeon is full" or "player is busy". Then you notice that your friend is doing a promotion quest - this is ok, then you can explain to them why you can't port and help them.
But, if they're menu chat the best you can do is say "Sorry I can't ", while they are sending out distress calls repeatedly. If promotion dungeons could have a warning saying " You are about to enter an area where your friends can't help you, " I think this would be helpful.