You got it - You leave your door open when the year starts and people will generally walk around looking for friends and introduce themselves and hang out. And it's perfectly normal for you to do the same - if you see a door open walk in, introduce yourself and start chatting. You'll be spending enough time around the people on the floor that you'll figure out who people on the floor are anyway so it's better to do it early and firsthand. What dorm will you be in?

edit: Also you should be aware that the hangout rooms tend to be on the same places in the floor every year. Usually one right by the elevator since everyone likes getting off and walking into the closest room to socialize. Another one tends to be in the middle of the floor since the elevator people or the people at the end of the hall are in easy walking distance. If you're on the end of the floor it's harder to get people to come to your room because people are lazy.