Multiple people can view an open apps script file at the same time,
but if one persons saves a change, the current instance of the file
that other viewers have open does not get refreshed. If one person
saves a change, they should immediately tell the others that they
saved a change, and to refresh their browser tab. Then you loose all
open tabs. It is very easy to overwrite the changes of someone else
if one person saves their changes. So, even if multiple people are
viewing, you should never have two people editing at the same time.
There is a Chrome extension that can be used to push/pull files to
GitHub/bitbucket.
https://chrome.google.com/webstore/detail/google-apps-script-github/lfjcgcmkmjjlieihflfhjopckgpelofo?hl=en