I suspect the reason they aren't letting us join the shadow druids is because they just won't matter. Everything about this act one seems to broadcast that the area won't matter once we leave it. We're never even told the name of the region. It's a place we're passing through to get information until we go to Moonrise Tower. From there, everything that actually MATTERS will happen in Baldur's Gate. Think about it. Every hint at the future steers us firmly towards Baldur's Gate specifically. Everyone leaving is going there, even the Hag circle seems to be based there. Everything of note in this area is gone by the time we leave, either dead by our hands, gone to Baldur's Gate or with its business already settled comfortably. And even the grove plot itself if you think about it. They want to close off the grove so no outsiders can get in. There isn't really a bigger plot we could join in on. They want to be left alone. Joining them would mean staying in the grove and not leaving, which means not participating in the rest of the game, which isn't an option thank to the tadpole in our brains anyway.
