NOTE: The answer above was for the 2016 website. Here is updated info for 2017.
The website has a new look for 2017. If you are working on your return, you can sign out by going to the left column menu and scroll way down to the bottom, where there is a "Sign Out" link. If you scroll the whole screen down, that may not be enough to see it.. You can scroll the left column menu separately from the main part of the screen.
