Any examples of ethical pyramid schemes?

A referral scheme and a pyramid scheme are very different things. A pyramid scheme is where revenues derived from recruiting people into the scheme exceed revenues from sales of the end product. Because people further up the pyramid make commission on the recruiting/sales of those further down, growth requires the perpetuation of fraudulent claims about income potential - selling the hope that the new recruit can reach those upper levels of the pyramid when it is more or less impossible unless you get in early, because people are already occupying the higher levels.

This all applies when the pyramid scheme is zero sum (i.e. the transfer of money up the pyramid). I once had the idea for a company that would do tutoring in a pyramid structure where a tutor would be one level above their tutee in this pyramid and that there would be non zero-sum interactions since the tutor also cements their understanding by trying to teach someone one level below them, with some monetary commission involved too for a tutor's tutee's tutee's tutoring, etc.

That wouldn't be a pyramid scheme by the aforementioned definition though.

You can't really have an ethical pyramid scheme by the definition, I think people just confuse "pyramid-shaped" things with the precise definition of a pyramid scheme.

I think the fundamental nature of a pyramid scheme is that the people below finance the people at the top. In the example you used, the people at the bottom are not injured by the people who invited them. It would be different if they had to pay a fee to sign up once invited that went directly to the person who invited them. That's the scenario that sets up the people at the base of the pyramid for fraud. In your example, people aren't injured if they can't find people to recruit.

Most jobs are positive pyramid schemes, IMO.