Perhaps offer an even BIGGER bonus when there are new players in the group... I don't know what the current bonus is, but make it something like 2x - 4x the normal award depending on how many new people there are. The more, the better...

How long are the longest cutscenes in these dungeons? Are we talking about lengths like 5 minutes or more? If so, pause the timer for X minutes and resume once all players are done with the cutscene, or X minutes has expired... If people are really concerned about losing those minutes. I'm sure the dungeon times have already been balanced with cutscenes in mind...