After joining the Dawnguards, and as part of the "New Order" quest, Isran asks you that you help recruit Gunmar and Sorine Jurard. Open our map and locate the destinations.

Recruiting Gunmar
Gunmar is located near Honeystrand Cave to the south of Riften. If you have Lord Tongue Overlook unlocked (SE of Riften), then you should fast travel there instead, as it is slightly closer. Either way, after fast traveling, continue on foot to Gunmar's location. Find Gunmar either in the entrance to the cave, or outside fighting some bears. Tell him that Isran needs his help. Gunmar is reluctant to help. Tell him you are up against vampires and that they have an Elder Scroll.

When Gunmar hears that the vampires posses an Elder Scroll, he agrees to help. But he does not want people to fall prey to the vicious bear that's stalking the lands. Agree to help Gunmar defeat the bear. Enter Crystaldrift Cave after him. There are many bears, as well as some Trolls in this cave. You are interested in killing a specific cave bear, which will be marked by an arrow. Kill the Cave Bear, then speak to Gunmar. He thanks you for helping him kill the bear and agrees listen to what Isran wants. Gunmar asks you if Isran is still at that fort near Stendarr's Beacon. Tell him that yes, to meet Isran in Fort Dawnguard.  Gunmar tells you he will meet you there, then leaves for Fort Dawnguard

Exit the cave.

Recruiting Sorine Jurand
Sorine Jurand is located to the west of Bruca's Leap Redoubt. Fast travel there (or to the nearest marker available) and proceed on foot (or mount) to Sorine's Location. Find her near some sort of Dwemer device. She asks you if you saw a sack full of dwarven gyros lying around. She suspects that mudcrabs might have taken it. Tell her Isran needs her help. Tell her about the vampires threatening Skyrim, and that the vampires are in possession of an Elder Scroll. But she needs your help. She wants you to find her at least one intact dwarven gyro. She tells you to either find the satchel those mudcrabs stole, or another gyro from someplace.

You can try persuading her that there's no time and she needs to get moving now. If the persuade is succesfull, then Sorine agrees to meet you at Fort Dawnguard. Otherwise, if you have a dwarven gyro on you, then you can give it to her. Otherwise, search for Sorine's Satchel. You should find the satchel pretty close to Sorine, near the river bank. In it you will find Dwemer Gyros. Return to Sorine and give her the Dwemer Gyro, and she agrees to help.

Your task now is to return to Isran. Fast Travel back to Fort Dawnguard.