There are 28 members in all to track down and are all over Hogwarts. I'll give you a couple to get started.
Dean Thomas is in the Transfiguration Courtyard. To get him to join you must find the 5 talking gargoyles. They are located at:
1. Going up the stairs to Defenses Against the Dark Arts classroom.
2. One in the Transfiguration courtyard right were Dean is standing
3. In the Paved Courtyard, up the stairs
4. On the stone pathway to the Viaduct
5. On the fourth floor. Go up the stairs to the left of the portrait. You will find it through the door on the left. Run all the way to the end of the balcony to find it.
Hermione will speak up when you are close to a gargoyle. once you have found all 5 go back to dean and he will head to the Room of Requirements.
Hannah Abbot can be found at the grand staircase. She will join if you can find a shortcut to the seventh floor. This one is easily done. Just go up the staircase to the right of Hannah and the portrait on the first landing has the password. Just ask and it will tell you. Return to Hannah to get her to head off to the room.
That should get you started. Just keep looking around all over and you will eventually find the others and learn what you need to do to have them join up. Good hunting!