Navigate to the repository page for the repo you want to collaborate on. 

Next, click on Settings directly above your repo description along the righthand side.

Select Collaborators from the sidebar on the left.

You can search for collaborators by their username, full name, or email address.

GitHub might be fun on your own, but working on projects is even more fruitful when you have collaborators to learn from and work with. Follow these steps to add collaborators on GitHub.