c4science is open for everyone to register to, by default an external user can use on of those 3 methods to create an account: Google account, Github account and Local account. The user will have to right to create repository, project or wiki page with those account types.
You can give access to those users to a project and a repository by adding the user in the Read/Edit/Push policies as any other user on the platform. It is preferred to use Projects to create groups to give access to user to a repository, see Project.
To sum up,
- Ask the user to register to c4science
- Add the user as a Project member that your repository use in the Policies settings
- Add the user to the users Policies of your repository
- The user should then have access to the repository