By using the git log command formatted and piped to sort -u (unique) you can pretty easily get a listing of who has ever contributed to a project which is under git source control.
git log --pretty=format:"%an" | sort -u
Using this you’ll get an output that resembles something like this.
Barry Sanders
Grant Hill
Joe Smith
As you can see, it sorts by the full name string which may not be ideal but I think that it does the job well enough for now.