If you're a Google Apps (now called 'G Suite') user, you can authenticate using your Google credentials.
Google Auth for Team Members (admins)
To setup Google Auth for team members, head over to the Team Members section and select the Single Sign-on tab.
Note: you need to be logged into Google already in order to enabled this setting.
Tick the box, and save the settings -- that's it. Now any existing team members can authenticate through Google.
Google Auth for Page Viewers
To setup Google Auth for page viewers on your Private status page, head over to Manage Account > Page Audience
Note: your page needs to be on a "Private" subscription in order to configure page viewer authentication.
Enter the domain of your G Suite account (you can enter multiple domains separated by commas if you want) and save your settings.
Now anyone who visits the page, will be required to authenticate using Google credentials associated with the domain(s) you entered.