For logins, you could disable registrations on the site and choose to manually create the users you want.. however, with your content blocked I'm not sure how you'd get people applying to register in the first place? Obviously this wouldn't matter if you're dealing with an invitation only group anyway..
Blocking access to the site..What you're asking is not a standard feature of ClassiPress or WordPress, however a quick search reveals there is a plugin for this kind of thing
http://www.cmurrayconsulting.com/sof...d-site-access/, although I can't confirm if it works..
Ultimately you probably could achieve what you want through customization, but as I said, these are not standard features..
