Well, yes and no. You can allow guest access for a SOLO, but the process is slightly different than with a 350 or 500. With the Slingbox SOLO, you will need to click on your email address up on the top right corner of the page on Slingbox.com (once you're logged in). Then you can go into your Slingbox directory and click "Edit", uncheck the box for admin passowrd and set your guest password. Then your guest can log into their own Sling Account and add your Slingbox with your Slingbox ID and the guest password. Or you can also follow the process below.
Here is an article from our support site with instructions on adding guest access to your Slingbox:
Hope this helps!
The Sling Moderation Team