Roof top parties are attractive for obvious reasons, particularly if there's a decent view, and especially on hot nights. Your considerations ought to go beyond the obvious - get permission first of don't do it. What can go wrong is known, because of what has happened in the past: people get drunk, fall off and die, people trip and fall and get hurt (must roofs have vent pipes and other obstructions, some have light wells to fall into, people can drop things off the roof, and cause damages below, or hurt someone, unusual use can damage the roof membrane causing leaks,
in the most extreme case a collapse could occur of the weight is beyond the structure's ability to support it.
You should cae, because your friends will sue you, or their relatives will when something innocent
goes badely wrong. If you think you have nothing to lose, think about all of your future paychecks
being smaller due to a court order each month. And think about when you GM or aunt dies 10 years from now, leaing you a big amount only for you to find that attorney has locked thatup as well?