When you return to Fort Dawnguard, you'll be attacked by a group of vampires. You'll have friends to help you. After you've killed them, talk to Isran. Isran will send you to find a couple of people to help with their battle effort.
Sarine Jurard is going to be out by Dragon's Bridge. Head there and then make your way to her. When you find her, talk to her and she'll tell you she's very busy. If your speechcraft is high enough, you'll be able to persuade her to leave her research and to return to Fort Dawnguard immediately. If not, you'll have to search the area for a Dwemer Gyro (if you don't already have one). When you find one, she'll head off to the Fort.
Gunmar will be out hunting in the Rift. Talk to him and he'll want you to kill a bear for him. This is quite simple since he'll be standing right in front of the cave. Go in the cave and the first thing you'll see is this bear. Kill the bear and Gunmar will be in the cave before long. Talk to him and he'll head back to Fort Dawnguard as well.
Head back to the fort and entertain Isran's spotlight. After that, head up the stairs and talk to him. He'll lead you to a surprise. Talk to Serana, then Isran, then Serana again. She'll tell you that you need to find a Moth Priest to read the scroll for you and the next quest will start.