Custom repository roles are now GA

Custom repository roles are now GA for GitHub.com and Enterprise Server 3.5.

Organization admins can create custom repository roles available to all repositories in their organization. Roles can be configured from a set of 35 fine grained permissions covering discussions, issues, pull requests, repos, and security alerts. Once a role is created, repository admins can assign a custom role to any individual or team in their repository.

Custom repository roles can be managed in the Repository roles tab of your Organization settings:

Custom repository roles are also supported in the GitHub REST APIs. The Custom Roles API can be used to list all custom repository roles in an organization, and the existing APIs for granting repository access to individuals and teams support custom repository roles.

To get started with custom repository roles, read the docs.