How to pass users back and forth using session data?

If you will work with sessions, then init this at first in your plugin, theme.

add_action( 'init', 'my_start_session' );

function my_start_session() {

    if ( session_id() )
        return;

    @session_cache_limiter('private, must-revalidate'); //private_no_expire
    @session_cache_expire(0);
    @session_start();
}

Alternative use the library from Eric Mann: WP Session Manager, also his tutorial.