Hi, Juline!
Great question!
We travel to Walt Disney World with large groups frequently, and I have to say that the Walt Disney World PhotoPass photographers are always so accommodating when we try to get two or three dozen of us in one photo!
We tend to agree on a meeting place and time amongst ourselves, then, once all have arrived, head over to a WDW photographer and let him or her arrange us all in a photo that's sure to be a group favorite. Photographers will be lined up along Main Street U.S.A., Juline, so pick an easy to reach location for assembly somewhere in Cinderella Castle forecourt. There, you'll have plenty of room for your group of 30 to pose and smile for the camera. (I have a feeling that this might take several shots until you get everyone focused; get ready for some laughs, Juline!)
If you think you'll want to purchase some of these wonderful PhotoPass shots, consider taking advantage of this
Disney PhotoPass+ preorder discount which will save you $50. For $149, you'll have access to all of your PhotoPass pictures, in addition to select attraction shots and photos from select dining locations as well. That price is accurate at the time of this writing, and you must order at least 14 days before your vacation. We LOVE that deal!
Have a wonderful time at Mickey's Very Merry Christmas Party, Juline; get ready for a night to remember...and the photographs to prove it!