There’s a plugin called WordPress eMember that may help you out there.
You can setup user roles like you want and it will protect certain content areas from non-logged in members. It’s very flexible in the way it controls access too: either a whole sub-directory, the entire domain, parts of posts / pages or comments can be protected using shortcodes. I believe it has an option to import a list of users into the database.
Would you set your users up manually or would you like to charge for membership access? Because if you do then you can use the eStore plugin from the same developer and charge recurring or one-off access fees to your members.
I’ve used both plugins and think they’re fantastic at what they do. The developers always get back to your questions, check with them to see if that plugin suits your needs.
Alternatively there’s the s2member plugin (free from the WordPress repository) which may also work – I’ve tried it once but it didn’t do the job I needed it for.