From a Unit Roster page or a Den / Patrol Roster page, click on the Show Settings button (gear icon):
The Scout Roster Preferences dialog box will pop up.
Check the box to Show Parents / Guardians and click Update:
Parents / Guardians will now be displayed with the last time they were invited to join Scoutbook or the last time they visited Scoutbook:
Please note that parents / guardians will still receive e-mails from Scoutbook even if they have never logged in (joined).