Just wondering if you become werewolf, will you get expelled from Dawnguard. Because werewolf is like Vampire that it is a curse/blessing from Daedric Lord, and Dawnguard is separated branch of Vigilant of Stendarr that hate all Daedric related things.

You can join them as a werewolf. Just don't transform in front of them.

but I think they should also react to Werewolf too


http://www.uesp.net/wiki/Skyrim:Vigilant_of_Stendarr#Vigilant_of_Stendarr

Player: "You hunt daedra?" "And any other abominations that prey on mortals. Vampires. Werewolves. Witches. But the Daedra are the worst. Their callous disregard for our lives is abhorrent in the eyes of the God of Mercy."

What they don't know, won't hurt them.

Well the Dawnguard test to see if people are a vampire by having sunlight come through the ceiling, lets hope they don't have liquid silver and start pouring it everyone. Besides the fact it can kill a werewolf, for it to be in liquid form the silver and the container itself would be scorching hot.