One of the main things that made WhatsApp popular was how it sucked in your phones address book and presented your contacts using the service. BlackBerry added a similar feature in BBM 10 that would scan your address book for potential BBM contacts (by BBID email I think) but they hid it on the third tab of the invites page. They also did not include this feature in cross platform BBM on Android and iPhone.
Not only should BlackBerry make sure this makes it into the next build of BBM for iOS and Android but they need to also make this a step for each and every user when they first set BBM up and every time a new contact shows up on BBM. It should check and ask you if you want to invite the contact.
This is functionality that BlackBerry already has but they really need to streamline it! Hopefully they listen and get this functionality out tomorrow… 🙂