Did you go to their HQ? Their chief is a little hostile but he recruits you anyway. He's in the main hall of the Dawnguard's HQ, you can't miss him. Also, there is a plot reason why they are not friendly. Give it time.
Hostile as in attacking you? That's a bug. Leaving and waiting for a few (in-game) days apparently helped someone, and I think I remember hearing that it might happen if you have a bounty, though I'm not sure. If it persists, your only hope might be calm spells.