yes, it's easy to make friends at work as well, just start hanging out with people that you kinda get along with. to be honest i have had very little friends all my life, i didn't really have a problem with it, and just recently found the importance of it. my problem was, i grew up in a bad part of town, and was like 1 of 4 white guys in my neighborhood. so as i started to realize it's important to have a healthy social network. i started making them, after you have a few, it gets easier. you can't just call the same guy over and over as your friend, you have to have a few. just find people you joke around alot with and can talk, be on the same page with. a dude that listens to mozart and discusses politics, will most likely not interest a headbanger that does drugs, and says f# america....,
just find people you "click" with and pursue the friendship. one of my new buddies is kinda "preppy yuppy" type, sometimes he'll joke about me kinda acting like a hillbilly, which, compared to him, i am, but we still get along really well though.
fyi, i was insanely introverted, as well. it's something you grow out of, as time goes on, but you have to push yourself. the most important thing is building confidence. if you're invited somewhere, GO TO IT. if someone says there's a party do you want to go, then GO! my problem was i would say no. then go play games on the computer. i was satisfied with it. but not anymore.
Edit: I looked back on here, and saw that some people try to be sort of a chamelion, like you have the stoner friends that act a certain way, try to copy them, then go to the hip hop guys and act hip hop. this is an extreme exaggeration but it's basically what is being said... I don't agree with this. if you don't like certain type of people, just avoid them. i've had enough troublemakers in my life, i don't need anymore. it's amazing how beneficial it is to meet QUALITY people, because they will also know QUALITY people you can also meet, so you will have a network of likeminded people.
good luck cesar, it takes work, but it can be done... just find some people you like, and when they invite you somewhere, DO IT! also, you can ask them if you can join them.