The rides were open in Tomorrowland, they checked for the wristbands. We arrived about 4:30pm and were able to use our MVMCP tickets to get in and got our wristbands. I believe if you are already in the park they have places where you can exchange your ticket for a wristband without going back out.
Thanks for the replies. We have been to the party before, but always just arrived just before 7:00. This time we are going to be there from early afternoon on.