Don´t know if you guys have come up with a solution to the 'guest access' issue with Slingbox M1, but here is what I did to add my M1 to a frends sling account:
First you need to get the Slingbox Id and user password for your M1. It can be obtained the same way as for the 350/500 models and is described here under 'Step 5'. (Look for the 'finderId' and 'userPassword' entries.)
Then go to http://accounts.slingbox.com and log in with your friends account. Click on 'Slingbox Directory' in the left hand menu and then click on the link where you are supposed to add PRO-HD or older slingboxes to your account. In the popup, just enter the Slingbox Id and user password. As the name of the slingbox I used the real name of my slingbox, don´t know if it matters or if you can type anything. Then just tick 'Remember Password' and hit 'Save' and you´re good to go.
Hope this helps anyone