I’m organizing a flashmob event - (b) present continuous for arrangements
It starts at ten in the morning - (a) present simple for scheduled events
How will we organize the event? - (d) will for predictions about the future
It’s going to be really hot on Saturday - (f) going to for predictions based on evidence
I hope that lots of people will be there - (c ) will for promises and hopes for the futures (often with hope, promise)
We’re going to leave - (e) going to for intentions
Some people may not understand - (g) - may/might for uncertain predictions about the future.