How do you build a community?

I am currently working on a product that is a social networking app. The app is about creating a room publicly or privately. People can jump into a room and share their thoughts, opinions in real time. Whoever creates the room by giving it a topic can set a duration for the room or close the room whenever he/she wants to. There are other amazing features like voting in the room e.t.c.

So my challenge is creating a community prior to launching it. How do I create a community?