We travelled JFK-FRA earlier in the year with my wife downstairs in Suites and myself and my son upstairs in J.
As others have said, my wife came upstairs to visit us in mid-flight and this was not an issue, she was escorted by one of the crew to find us, we did not attempt to go downstairs to visit her and I am sure this would be frowned upon, there is a velvet rope across the top of the stairs and crew lurking at the bottom.
What we did find useful was the seat to seat phone calling. However both users have to enable incoming calls for this to work, obviously you also need to know the seat number of the rest of your party.