A typical mistake that people make when writing their sites is to make their most interesting content only available to members who register on the site and who then need to log in to access it with a username and password. Unfortunately, this doesn’t make it accessible from a Search Engine perspective.
From a commercial point of view, it might be appropriate to do this – after all, if you have great content that you want people to pay to access, you need to protect it from those who don’t pay. Where it makes less sense is where you don’t charge for the content, but you want to collect information from your website visitors before allowing them access to the content. From a search engine perspective, you might want to consider this very carefully.
When search engines come visit your site to index your pages they cannot log in to get access to all of the protected areas of your site. They will only see the commonly available content, and so only this will be indexed. Think about it – if you have some great content that is hidden behind a login (or other protected) page, search engines will not know it is there and so will not be able to return it in a results page. This could mean you are missing out on a valuable traffic source, unless…
…you engineer a compromise solution for your website. One such solution might be to make a snippet of each page available to non-logged-in visitors, where they have to log in to see the entire page content. As long as you include a sufficiently compelling title to the content and a good smattering of keywords in the leader text, you will have a good chance of attracting search engine visitors to your site (and if your content appears interesting enough, may encourage those visitors to register to see the whole page).
Hint: It’s a really good idea to engineer this from the start, rather than try to change things later.