To be fair these expectations vary wildly between cultures and countries. Where I come from the starting time of a party/event is regarded as actual starting time. You’re welcome, not expected, to arrive at that time and while there might be some minor preparations left, you’re in no way expected to help by being on time.

However there was a reddit thread (or was it HN?) a few weeks back discussing this very issue and if i remember correctly being on time was considered almost rude in brazil as well as a number of other countries,