To control who can log into your Chromebook, open your settings as follows:
  1. Click your account picture on the shelf.
  2. Click "Settings."

Once in settings, do the following:
  1. Scroll down until you find the "People" section.
  2. Click "Manage other users..." button.
  3. Modify your settings as follows:
    1. Uncheck "Enable Guest browsing"
    2. Check "Restrict sign-in to the following users"
    3. Add the user accounts that you wish to allow access to your Chromebook
    4. Press the enter key
    5. Click "Done"