I have a question.

I'm my band's sole financial contributor. It's my project, my songs, and
everything is recorded in my home studio. However, in one exceptional case,
I was on a deadline and needed very specific types of back vocals, which my
guitarist volunteered to help me with.

After I explained to him exactly what I needed in detail, he went out and
found a couple of backup vocalists, taught them the 4 lines they needed to
sing, and recorded them. He then emailed me all those raw takes for me to
mix, and we're going ahead with them.

I'd like to know what proper crediting protocol for this sort of thing is,
and I'll endeavor to follow convention.

Thanks.