git is distributed. You can give access to a mirror of your git repo for execs that don't access the repo or contribute to it. They don't need to be github users, they just need git access to an url. If github is still convenient for you, there is a solution to avoid paying for this users. It only really makes sense to have developers added as github users (of course, it used to be convenient to just ask them to be github users, but it never really made sense)