Protect area of website with password

I’m converting an existing website into a WP backed one. The current website has a ‘members only’ area which is protected by a login/password using htpasswd. People log in once, and they can visit that page plus its descendent pages. All members share the same login and password which is given to them with their membership. I haven’t been able to find a way to do this with WordPress, if possible at all.

I don’t mean the password protection per page that is already standard in WordPress.

Any suggestions?