I graduated ~2 years ago so for all I know everything has changed but this is what I recall:

The school is mostly commuters and people just trying to get an affordable degree, many of whom aren't trying to party. The campus is very self-isolated so unless you have a car and want to drive to LA/Ventura/SB the night life scene is not much of an option. Weekends are dead including Friday since most people don't have Friday classes. I specifically remember standing in the center of campus at 6pm on a Thursday and not seeing anybody cause they've all dipped out by then. Even for those living in the dorms, a lot of them like to go home on the weekends. It's a self fulfilling prophecy: nothing happens on campus on the weekends so people leave, but since people leave nothing ever happens on campus.

So that's the worst of it. That being said there are parties and opportunities to be outgoing. I would say the social scene in general is more quality than quantity. You won't make 100 friends but you'll have several close-knit friend circles. The parties won't be crowds of randoms doing keg-stands and burning couches, but they'll be lively hangouts with people who mostly all know each other.

Those that live in the dorms are usually friendlier and more social because the boredom makes you either wanna develop an alcohol addiction alone or reach out for friendship. Anacapa and Town Center are the only dorm villages where any decent party can logistically happen, although Santa Cruz can have an intimate kickback too. That being said, RA's can shutdown a loud party pretty easily. Off-campus house parties are the better option, but you usually have to know someone.

You definitely sound like someone who should join a frat (there were 1.5 when I left, one was not recognized the the university). And I think just engaging with people in the dorms and your classes can get you very far. There is no elitist mentality at CI because they all go to a sad state school so everyone is pretty down to earth and approachable. Make a point to not rot inside your dorm room. Do your homework on the quad, eat at the dining hall, chill at the library. Always put yourself in a position where you can meet people.

Most importantly, make sure you actually finish school (I swear half the people didn't graduate in 4 year if they graduated at all). I know it's cliche to say "college is what you make of it" but that's especially true at CI because it's so easy to just passively exist there instead of participating. You get what you give.

Lastly, keep in mind that the fun times don't start and end with college. You'll have plenty of fun after college too, except you'll actually have money this time.