How can I give user access to areas of an ecommerce site without having to authenticate

We have a large ecommerce program where we can make different shopping carts for our customers.  We then create login and passwords for them to use.  What options exist that would enable me to distribute a link to a page inside of a customer site so that the customer could click it and get access without having to authenticate.  For example, say I want to give a group of  VIP's a single link to come into their site and look at inventory.  The link would be part of an electronic file ( a pdf).   While they are reading the PDF they could click the link and look at their inventory page- no passwords, etc.  Just a link in.  Can I do that somehow if someone did a script of or something? Like a user and password does exist- but somehow would get entered automatically by the script.  The software has no unprotected areas of a customer site, everything is behind the authentiction.  Just thinking out loud... but if anyone has any ideas- I am all ears!