Login/password integration

I blog for an organisation that is currently looking to revamp their website. They are looking to add a login/password to all parts of the website including the wordpress blog. So i need to know if it’s possible to integrate the blog with the the rest of the site in this respect. Making sure that a user would have to login to see the blog and the login would stay valid when they navigate to the main site and vice-versa.

Cheers

E