Are you sure you want to do this? It is usually not tailor-made unless very large projects that need multiple people administering (collaborating with) it. In small files, you should not have multiple repository administrators, otherwise it will be a zone.
Anyway, only one account can be the owner of the repository.
If you need multiple people tweaking maybe you have multiple repositories in one, and most certainly should be separating them. Github does not encourage the use of an all-in-one repository.
The right thing is for people to pull request , the famous PRs. Then it does a branch and at some point asks the administrator / owner of the repository to accept the changes that it made. This is the correct process, and Github has the right tools for it. I suggest learning them.
But if you really want to put other people as an administrator, which is not to say that you do not need to do PR, you can follow the documentation :
Settings - > Collaborators - > and go there with Add Collaborator.
See more .