Interesting. I hadn't made this connection between BG I & IIs Shadow Druids and this 'Shadow Circle', although it looks like Larian has. Some evolution or offshoot?
Still, I think it would be possible to work alongside them, as that blurb you posted indicates they do work alongside 'less civilized' groups and individuals rather readily, and of course, Faldorn and the shadow druids could be allied with in BG I & II. Now 'joining' them, that's obviously another story. I'd suggest to treat it more like the Zhent interactions are treated in the game right now. You do their leader a favor and it's hinted at that it will evolve into a more fruitful relationship later on (presumably in act II) You don't formally join the Zhents, but you do help them, they reward you, with the implicit expectation that it will lead to something more down the line.
So perhaps you help them out with their grove problem and they might provide enough help to point you in the right direction like Halsin would normally, but it also puts you on their 'radar' as a future ally, and they might consider helping you against the absolute down the road. Druid characters might get a formal invite down the road if they continue to show promise.
