Retrieve User Data Outside of Core Blog

My set up is as follows:

I have a regular website. In the blog/ folder, obviously, contains my blog. I would like to be able to use the WordPress user system across the board, but I’m not sure how.

For example: User is logged in to the blog. They go check out the main site to do a search for homes. I only want to show the “add to Favorites” or “save” to those users that are logged in. I don’t have any trouble with this in the “blog” folder. What file…..or functions….or anything…do I need to include on the main site to get this to work? I’m already using the <? include ‘./blog/wp-blog-header.php’; ?> for grabbing the first post from the blog to put on the home page.

Any and all help is greatly appreciated!