Hi, if your whole website is for members and behind a login and password, Searchengines can't index the website and thus not visisble for others than your members.
if you want other people to find your website, you'll need a public part, which you can optimize for your users and searchengines.
the question is: do you want other people than your members find the website, if yes, than you'll need content that searchengines can find. If the answer is no you can hide the whole website behind a login and password.
i manage a website which a part of that is only for members. that part is not optimized and behind a login and password. The rest of the site is public and need to be found in the searchengines. This part is optimized for on - and off page seo.
Grtz, Leonie