If you log into your website as owner/admin, you can go (from the overhead menu ribbon) to Tools > Privacy. The first section of the page deals with the privacy type setting (explained in the box to the right, "control access to your private information). You can use the recommended membership model with invitation or choose to use an access code. [if you don't see the "Tools" link, you might be viewing your website as family member: in that case, go to Edit > Admin Log-In]
It appears from what you say that you may currently be in this latter setting. If you click on it, you should be able to change the password (code). You may consider switching to membership (invitation) model, also. If instead you are already in membership model (which, I assume, is the default setting), then there is no particular password your guest needs to input, save for their own one for their account (they must come up with one when setting up the account, then use that same one to access the website).
Hope this was of help!