In a small weekend update, +Google has added a new option to the notification settings.
The new option allows you to select whether you'd like to receive either an email or phone notification when someone joins Google+ who is already in your Google contacts list. It may be a good idea to have the option checked, so when your family/friends/coworkers do get round to joining, you can give them a big welcome!
Where Is It?
You can find the new option listed under "Circles" when visiting the Google+ settings page, which you can access by clicking here.