First things first: try to break the ice or start a conversation. Start out by trying to find people you have things in common with and people you enjoy talking to.
When you are walking around the office, take a look at what people have on their desks. Pictures, awards, trinkets, etc. If someone has a football team flag and you know remotely anything about football, make a casual comment along the lines of "Hey, [your team name] had a great game last week! Have you been a lifelong fan? Are you originally from there?". If they have a picture of the family and kids, ask about how old they are or if you have kids, maybe they go to the same school or there is a school rivalry. You are looking for anything you have in common to open the door for a conversation.
Another thing to keep in mind is that people are busy. Especially people with children and active lives outside of work. If you are a younger dev or a person with no kids, chances are you have way more time than people who have to worry about getting a bunch of kids to various school events, after school stuff, etc. Those people may not have time to go catch a movie or go to happy hours.
Just be friendly, find some people with similar interests, and take baby steps. Start with a casual chat at the watercooler, move it up to going to grab lunch together, and then work it up to a after work happy hour. Don't expect to become best buds or inseparable after only a couple weeks.