You have to contribute something very significant or have a game changer.
When I was a medical student, I made my own notes for a particular subject. I got in contact with FA to try to incorporate my stuff into it. They were very willing to take a look at it, and make me an author.
But.... they said I had to wait a year. The new version of FA wasn't out yet, but was already printed.
So I brought my stuff to LWW.