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.

Type the name of the collaborator you want to invite to your project.

When you find them, just hit the green button to add them to this repository. They will get an invite momentarily. Once they accept, you can collaborate on your repository together.

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.

How to Add Collaborators on GitHub