As far as Pavak's joining...my best guess at this point is that he's traveling through the area on his way towards the Korcari Wilds perhaps in search of Chasind. He would have preferred to stay in Denerim, being a total city-boy himself, but after a close call with a Templar he decided instead to perhaps seek out some of these Chasind apostates that Thren told him about.
How exactly the group meets him, or when, is totally up to you guys. I could see him making some total city-boy blunder and getting into trouble out there in the Bannorn, or perhaps he's skulking around in bear or spider form figuring he's safer that way and he gives the whole group a scare. What do you guys think would work best for you?
I figure his motivation for joining with you is two-fold: for one, he's got nothing else to do. I mean, he's flailing about the Ferelden backcountry with no direction or friends, lol. And two, he can hardly turn down any kind of challenge - doing something as crazy as fighting darkspawn would get his interest piqued for sure!