y not: It is said that there were millions of devas and brahmas there to listen to that discourse, and many of them attained various stages of magga phala. Of course, a human would have seen only the five ascetics there.
– And it is said that (I cannot give a reference offhand. It is probably in the Vinaya Piṭaka) many devas can fit into a space of a hole in a needle. It is not possible for us to comprehend these things.
Those higher beings are not visible to us, but if they want they can show themselves (they can even take the form of humans).
The Buddha did not have to assist them, even though he was of course aware of them (but probably not the five ascetics).
In a related story in the Tipitaka is the following account: One of those “unseen beings” explained the Dhamma that he learned from the Buddha to another who had not heard it. Their discussion took place over a house where a woman by the name of Kali was feeding her baby at night. She “heard” their whole conversation and attained the Sotapanna stage. She is supposed to be the second human who attained the Sotapanna stage that night (Ven. Kondanna was the first).
– Now it was not the case that those two beings conversed in a human language. It the the “sanna” that she was able to perceive; basically the ideas. Again, I am not sure exactly how that happens. The closest analogy I can think about is how we “know” what people in our dreams say.
More details at: HEMAVATA SUTTA by Ven. Mahasi Sayadaw.