Welcome to our Community and thanks so much for your first post to our wall - I am sorry that there are some issues with signing into your Hub. If you feel that someone else is signed in already - the best thing to do is to do a PIN hole reset ASAP.
This will reset your Hub to the factory settings that can be located on the bottom of your Hub. After the PIN hole reset (hold for around 60 seconds to just make sure), you can then log back into your Hub via the address on the base (depending on what hub it is) and change your passwords as soon as you can.
You can check this page for changing your Hub password.
We're unable to assist you with who may be logged into your Hub I'm afraid - I am sorry.