Simply open the BBM app on your BlackBerry 10 phone. Tap on the navigation menu and select 'Invites'.
You'll see tabs at the top of the screen, one of the options is called 'Suggested'. Tap on that.
On this page, you'll see a list of people who have used BBM that are also in your contacts list. Just tap on their names to send a BBM invite and you're sorted!
This is quite handy as I've found a few people who I didn't even realise had a BlackBerry. However, do bear in mind that it might match people who have used a BlackBerry handset, but aren't any more.
Of course - with BBM going cross-platform soon, this might become a very important page!