How to add collaborators to a GitHub repository
Step 1: Go to settings on your repository
Step 2: Click on “Collaborators”
Step 3: Then click on “Add people” to add collaborators to your GitHub repository, and add them by searching for their GitHub username or email.
After above steps, those who you have added will receive an invitation email with a link to accept the invitation to the repository. After they accepted the invitation, they are successfully collaborators of your GitHub repository.