The most popular and largest groups would be the football and basketball teams. Howerver, as you can imagine, with 45000 kids from all over the world, there are clubs for any sport you can imagine. Some of the largest organizations are the religious organizations who have been for the most part very tolerant of someone like myself with a complete lack of spirituality.
My favorite group is not an official organization, but an offshoot of SCF (spartan christian fellowship) who play ultimate frisbee twice a week. It is just a group that likes to have fun adn run around chasing a disc.
Most people like to leave their doors open, but it really varies from community to community. It depends a lot on the demographic of students on the floor, and how close people get during welcome week and the early parts of the semester.
Athletic events are wildly popular. On a saturday afternoon there are people spilling out of cars and dorms all over the place heading for spartan stadium. Guest speakers and meetings are posted in the cafeterias andon bulliten boards, along with theaer and comedy performances in the Wharton center. Outside of that, there are movies every weekend (Friday, Saturday and Sunday nights) that have just gotten out of the theatres but are not yet on DVD. These are held in lecture halls and are free (FREE) to students in the dorms.
45000 people with a pretty even split from all over the world results in lots of options for either gender. There are so many places to go, from restaurants to movies to sporting events that anybody with an ounce of creativity can come up with a nice and inexpesive outing with their significant (or prospective significant) other.
I met my closest friends jsut by chance through the wonderous sport of frisbee. I was walking back from class one day after taking a particularly awful test when I saw a disc flying through the air and asked if I could play with them. It was the start of friendships that will most liktely last for many years, and it is a bit of a microchasm of my experience at MSU.
Im usually not awake at 2AM. If I am, I'm likely procrastinating, or playing frisbee with glowsticks and a luminescent frisbee (my favorite pasttime).
There are all kinds of traditions at MSU. I guess they just develop after a university has been around for so long. Football season and tailgating come to mind, as well as midnight scream during exam week (I think the name is pretty self explanatory), as well as Spartywatch during the night before the UM game just to name a couple.
People party a lot, way too much in my opinion. If that is your scene (it is not mine) then you are going to have to get that information elsewhere, but I hear its not a difficult thing to find. Just keep in mind there are other things to do.
Fraternities and Sororities play a large part in some students college experience. Personally I am a bit removed from that part of the university, but these organizations do a lot of things such as community service that are admirable, even if they often get a reputation as just hosting lots of parties.
Last weekend I went to Shenandoah National Park to hike on the Appalachin trail. I got an internship here in Virginia for the summer thanks to some professors at MSU, so I am not going to be at MSU until the fall. However, a typical fall or spring weekend for me would be a saturday of Mountain biking, probably at Pickney state park ~1 hour away, followed by an afternoon of watching football at Spartan Stadium or playing Ultimate depending on the season, followed by a sunday of more Ultimate, doing laundry, and then watching a movie in one of the lecture halls.
On Saturday nights I am usually pretty exhausted from the day. I am very outdoorsy, so I am usually beat by the time the sun goes down. However, I would say the most common thing that people do besides drinking on saturday nights is go watch movies. There are usually 3 choices each weekend every night at 2 times around campus. There are also periodic concerts hosted by the residence hall association. It is not hard to find activities provided you aree willing to look.
I do a lot of biking off campus, as well as play a lot of frisbee and hang out with friends who live of campus. When I have family around, I escape the dorm food (which isn't bad, just a little monotonous) and go to one of the restaurants around the area such as Old Chigcago or Iggy's Cafe.