So they'll take a vampire hunter stupid enough to waltz in with the leader's daughter? And they seem fine with letting random npcs join as "spies" why not you? Here's an easy way to make it make sense:
Vampire hear's about the castle, finds it thinking he'll get in. Volkihar cast a spell on you that makes you black out. Wake up in torture room being interrogated. You say you want to join. They laugh and send you on some suicide mission. You come back and they're impressed. The story goes on from there.
Nice, I like it! It makes way more sense than it does in the actual add-on.
Hell, when Dawnguard's trailer first released, I thought that both HQs would have been there from a rumor, but not that you were forced to go firstly only in one, though. One should be free to either join already the Vamps or the Dawns whener they feel like it, because I know that many players roleplaying a vampire felt frustrated when they found out they were forced to go visit the vampire hunters!