If your account is designated as the owner of your Chromebook, you can control who can sign in. Using the settings option you enable/disable the guest mode, or restrict the device to specific users.
To change your Restrictions passcode after you turn on Restrictions, follow these steps:
To see a list of apps and features that you can restrict, go to Settings > General > Restrictions. To turn restrictions on or off, tap the slider next to the app or feature.
If an app or feature is on or if there's a checkmark next to it, anyone using your device can use that app or feature. Otherwise, no one can use that app or feature on your device.
Find out more on the Apple Site.