Hi, with the slingbox M1 you can not add slingbox ID's to different accounts, only the Solo and Pro-HD have that option
The slingbox 350 would be i think the best option for you as it gives you the same video quality as the M1 and allows you to add a guest user then the sling account of your friend will have access as guest while you will be the admin
A while ago I posted a description of the steps I performed to add my Slingbox M1 to a friends account.
As long as you use the 'userPassword' when you add the M1 to your friends account it should work as before in terms of you being able to "kick him out" since you are the admin of the slingbox.
It was a while ago and I haven't tried it since but hopefully it will still do the job for you.