If you're using GitHub Free, you can add unlimited collaborators on public repositories, and up to three collaborators on private repositories owned by your personal account. To add more than three other people as repository collaborators, upgrade to GitHub Pro. For more information, see "GitHub's products."
You can give people collaborator access to repositories owned by your personal account.
You can invite users to become collaborators to your personal repository.
When you remove a collaborator from your project, they lose read/write access to your repository. If the repository is private and the person has created a fork, then that fork is also deleted.
If you no longer want to be a collaborator on someone else's repository, you can remove yourself.