The only way to access a Slingbox M2 is to use the login information for the Slingbox Account it is linked to. It (as well as any other Slingbox) cannot be accessed by multiple computers/devices at the same time.
The last Slingbox that required/allowed you to have an administrator password was the PRO-HD. Slingboxes that came after that do not have any way to put in an admin password. In this case, the best thing to do would be to make sure you have a secure Slingbox account password that you share with as few people as possible.
Hope this helps!