First of all - lucky you. I was wishing that was the case when we had our DP in Epcot in November 2008. I have also heard that guests can stay if there are EMH. Not sure how they would work that. Our DP was in the UK and everyone had wristbands that were given out at the wedding so they could get into the parks without a ticket. Our guests met a CM at the front of Epcot and all walked in to our DP location. When the party was over they were not walked out - everyone could just leave when they wanted to so I see no reason why they could not stay for EMH.
Back then you did not have to show a room key (I don't think) for EMH but you do now if you want to go on any rides or enter any attractions for EMH, so as long as your guests are staying on Disney property and have a key with them there should be no reason they could not stay. How are they all getting to your DP? I am sure Carrie or someone with more recent knowledge will chime in here but I just wanted to give you my thoughts.
Linda