Git does not have a integral mechanism to have multiple authors per Git commit. A Git coauthor notation convention has become accepted by major services including Github and GitLab.
Indicate Git coauthor by placing plaintext in the commit message body. The email address cited must match a registered email with the Git service. The email can be a working public email or the “fake” noreply email provided by the Git service. Multiple coauthors each use the same syntax on the same Git commit like:
added foo function to bar.py Co-authored-by: David <email@example.com>
On GitHub, a coauthor commit looks like:
Currently, the coauthored commit doesn’t count on the user’s GitHub contribution calendar.
The coauthor commits do show up in GitHub search under “Commits”.
Caveats: as with regular Git commits, there is no authentication to avoid someone masquerading as someone else with Git coauthor commits. Git coauthor commits cannot be GPG signed for each coauthor, only the primary Git committer can GPG sign as usual.