I am the author of an open-source program. I am the primary author, and the repository on github is owned by me. There are however, ~3 other individuals who have made a few dozen commits into the project. Their commits are non-trivial, and cannot simply be removed.
I have a company that wants to pay me to modify this program to fit their needs. However, they want the changes to be closed source. Part of their business involves them distributing this binary to their users via a web assembly. My understanding is that since they are distributing this binary, they must comply with requests for the source.
I would like to re-license a fork of my project. My understanding is that I can obtain permission from the ~3 other individuals and create a fork.
My question is: What is the process? Is this as trivial as receiving written permission to fork as of a specified commit? Or is there a deeper legal process involved?