Are you sure you want to do this? It is usually not suitable unless very large projects need several people managing (collaborating with) it. Small ones should not have multiple repository administrators, otherwise it becomes zone.
Anyway only one account can be the owner of the repository.
If you need a lot of people messing around, maybe you have a lot of repositories in one, and you should probably separate them. Github does not encourage the use of all-in-one repository.
The right thing is for people to do pull request, the famous Prs. Then she makes a branch and at a certain point asks the repository administrator/owner to accept the changes it has made. This is the right process, and Github has the right tools for it. I suggest learning them.
But if you really want to put other people on as an administrator, which doesn’t mean you don’t have to do PR, you can follow what you send to documentation:
Settings -> Colaboradors -> and there go in the box with Add Collaborator.
See more.
"upar"?! " upasse"?! "upar"??
– José Diz