In our experience, the parks are extremely crowded in that 5-9 period. And, sometimes, depending on the security team working that night, the crowds stay pretty bad for an additional hour or so after official close. For our last meetup this past November, the first night was crowded most of the night, and we discovered that regular park guests had been hiding in the area around one of the water rides in US that was closed but not closed off. We reported that to the OI team and noticed that the area was actually roped off with security guards blocking it the next night. Huge difference!
The event is great, though, and you get hotel discounts in addition to free food etc. You can math out the hotel savings and how much food/butterbeer you think your family will consume and see if the math maths out for ya

One year my group counted the butterbeers we drank and it was...scary lol