Thank you for your answers !
@YCrepeau you are absolutely right.
The basic problem is that ghost has not yet implemented a special authentication system, and that I have my community on Dicourse.
I may choose to place the reserved content on the forum, rather than the blog, and use the groups.
If I want to offer special content on the site, and identify the members of the forum that uses it, or simply avoid them registering on 2 different systems : I could find out about the discourse sso. But allowing a member of the forum to write an article on the blog will always be impossible (waiting for Ghost sso is maybe my solution ?)
Its on the roadmap :