Sadly, the Bon Voyage party is no longer allowed on most ships. I say most because I have not sailed on ultra luxury ships. The majors such as Celebrity, RCCl, Princess and HAL to the best of my knowledge allow only passengers on their ships prior to sailing.
Here is an idea...meet your friends at a restaurant or bar the noon you are scheduled to cruise. Buy everyone lunch and drinks and then board the ship. I know that it isn't the same as a party on the ship...just an idea for you.
Hope that yu have a great cruise.