Yes, I think we are good to close. It is indeed very possible to invite people vho then log in via SSO or whatever login method the site provides. Depending on how the invite is setup, the invited user can be added to groups and dropped in a specific topic as they log in. Pretty sweet! 
Most of the discussion here is actually about SSO support for adding and removing a user to groups which I am less familiar with and do not see much documentation about here on meta. Someone more familiar could write a FAQ about it to pull together what is known about it. It allows users to be added or removed from groups as they log in via SSO, which actually has nothing to do with the invite system.
This is a good example from @simon for how to do it using WordPress: Automatically Adding New Users (from WP integration) To A Group - #4 by simon
There are caveats, however, so maybe it’s not worth encouraging people to implement this method. The Discourse groups have to exist already for this to work, and it requires the user to log out and log back in again for any changes on the auth provider end to take effect.