At 10, there are only 8 people who are willing to join. This suggests that if 10 are expected, the event will die.
At 20, we have 22 who are expected and so the event will grow.
At 50 we have over 60 and so it keeps growing.
At 60 it is about 68.
At 90 there are only about 88 willing to join so if 90 are expected, the event will shrink.