> My recommendation would be to have a NewGroup function or equivalent that returns an empty group

That goes against common practices in Golang, articulated in the second paragraph of https://go.dev/doc/effective_go#allocation_new among many other places.

Also the errgroup documentation specifically says "A zero Group is valid, has no limit on the number of active goroutines, and does not cancel on error." And, as you noted, one of the examples doesn't use WithContext.

That’s one of my Go pet peeves - zero/default structs are sometimes valid and sometimes not, and the only way to know is to dig into the docs. I prefer the API to speak for itself, ideally enforced by the language/compiler.