For iOS:
To get notified when address book friends join textPlus, tap the Address Book ON|OFF key on the Settings screen so it reads ON. To disable address book notifications, tap the key so it reads OFF.
If your facebook account is linked to your textPlus account, you can do the same for facebook notifications by tapping the Facebook Friends ON|OFF key.
For Android:
To get notified when address book friends join textPlus, tap the Contacts Notifications check box on the Settings screen so it has a check mark in it. To disable contacts notifications, tap that check box so it is empty.
If your facebook account is linked to your textPlus account you can do the same for facebook notifications by tapping the Facebook Notifications check box.
For WP7:
To get notified when address book friends join textPlus, tap Facebook Friends in the Account Setup page and flip the switch to the “ON” position. To disable contacts notifications, flip that switch so it is OFF.