Ho man, so much reading (and no, shamefully, I just skimmed)...anyway, I'm thinking the best way that Arya would be recruited would be that she's run into a darkspawn or two and is like "wtf is this." She starts taking to the trees more to scout out areas and happens upon the group headed for Lanaya's clan? Hears them looking for Dalish and thinking better of traveling alone in the woods that have suddenly become much more dangerous, may even perhaps approach them?
I'm still trying to work out if Terin made it to the Dalish or not, but I'm starting to lean towards the idea that either a) he didn't, to leave it open for a sidequest for her later on, or b) he got caught up in the fighting and was killed, but it seems kind of sad to leave Arya completely stranded like that. Should obviously make that decision soon.
OR. Arya could be found in conflict with something, having to defend herself (darkspawn?) and the group helps her out? These are my suggestions going off of what I've gleaned from this post, so please feel free to correct me or make suggestions on my suggestions.