One Unix account for several users on GitHub, how does it work?
Solution 1:
We're using our own GIT repo, and we achieved this with multiple records in authorized_keys. The key point which is confusing you is that the git account does not have direct access to a terminal, but is using the git program. Internally, each key is recognized as particular user, which have particular access options, and the git itself is managing this. It is configurable.