I bet that they welcome werewolves into their ranks as they'd be a great counter to VampLords and aren't a burden on the supply of Cure Disease potions, but they won't all be werewolves.
I'm not sure about that. A werewolf is too much of a wild card, a dangerous beast that can't be controlled. If there were some werewolves in the Dawnguard faction, and they happened to change in the middle of a battle, both the vampires and Dawnguard would have trouble on their hands. Werewolves aren't
that much better in human form to be worth the risk of their volatility when changed.
That's not to say there couldn't be a werewolf or two in the Dawnguard, but it wouldn't be something known to the faction as a whole.