We had a report of this about a month ago and we were sent in a screenshot of it in action. However, being a screenshot it was hard to tell if someone had disconnected and the party on the left of the screen had simply not updated. Another report was made in the Beta forums about this bug and it apparently does happen and is still happening.
Wellsey posted a link to his video which clearly shows five players in the game at around 2 hour and 3 minutes on the video. The Blizzard team are obviously on the case, responding to the thread. It seems this may be happening more frequently as another post appeared in the bug forum shortly after.
Blizzard are asking players to report this bug and send get a copy of the D3Debug.txt file over to them so they can try and nail the problem down.
I know some people would love to see the party number increase but it’s not going to happen, neat to see it in action though.