If you are working with someone and you would like to have a co-author it is easiest to do from Atom (text editor), but it can also be done from the git command line. Note that there is a NOT a ending quotation mark after the description of the commit message
git commit -m "Add test to model.
Co-authored-by: name <email@example.com>
Co-authored-by: another-name <firstname.lastname@example.org>"
If both authors have GitHub accounts both will be assigned the commits.