You can invite users to become collaborators to your personal repository.

Who can use this feature?

If you're using GitHub Free, you can add unlimited collaborators on public and private repositories.

Inviting a collaborator to a personal repository

You can send an invitation to collaborate in your repository directly to someone on GitHub, or to the person's email address.

Ask for the username of the person you're inviting as a collaborator. If they don't have a username yet, they can sign up for GitHub. For more information, see Creating an account on GitHub.
On GitHub, navigate to the main page of the repository.
Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.
In the "Access" section of the sidebar, click Collaborators.
Click Add people.
In the search field, start typing the name of person you want to invite, then click a name in the list of matches.
Click Add NAME to REPOSITORY.
The user will receive an email inviting them to the repository. Once they accept your invitation, they will have collaborator access to your repository.