Think like a tribe and do things to strengthen the tribe. A culture is strengthened by rituals and mutual goals, so do things that reinforce that. Divide your culture into three areas: 1) Work, 2) Play, 3) Philosophical; and do things that will reinforce those. Here are some specific recommendations:
Work
Adopt development methodologies (like Agile) stolen/gleaned from well-documented sources. Adopt specific syntactical coding standards so that everyone's code looks identical. Do not permit anyone to use tools to make their code "pretty". Have inclusive meetings to hash out the specifics and get complete buy-in from all team (tribe) members. Document well for future members. More specifically, institute weekly code reviews (at least until they start becoming pointless because everyone is finally on the same page) where one person's code is examined and discussed. This is a ritual that exposes egos, reveals misunderstandings, and exposes weaknesses that can be remedied. After a few cycles through everyone in the team, this will become less difficult and there will be far fewer issues found. Pro tip: start with the tribe leader's code. New members will view code reviews as a kind of initiation, which it is. I was on a team of five developers that did this and it did more to bring us together as a tribe than any single thing. Caught tons of bugs well before they were integrated into the rest of the system. (Initially, we could not check code in until it had been reviewed.)
Play
Eat lunch together as a tribe at least once a week if not more often. Eat in the office and play poker, (not for money) if possible. Always invite everyone in the tribe and if you have a lone wolf that never eats with everyone else, have his boss start holding work lunches, requiring attendance (and then just don't do any real work). Similarly, have some kind of family friendly weekend, off-site event at least once a quarter, paid for by the company if possible, such as cookouts, bowling, sporting event. Ideally, this will be more picnic-like than movie-like to foster getting to know families. I was part of a team that did this (not the same as the one mentioned above). Picnics, a softball team, a volleyball team, movie days (we'd take a long lunch and all go to a new release) were all part of it. It was great until we merged with another company that took over, trashed our budget for such things, split the tribe, started having conference calls during lunch time. Tribe hung on for a while but once these changes were in place, it was clear to us that the company that took us over had no soul.
Philosophical
Have regular "meetings" to discuss technology trends, ideas, stuff found on the web. A lot of this will happen at those regular lunches but try to let lunch be more social, less work talk. As individuals, investigate new methodologies and tools to adopt and then discuss them in a group. Find out what websites each of you regularly reads. Tell war stories from previous jobs or college. Think of these meetings as the time to plan how to strengthen the tribe. The atmosphere of these "meetings" should be akin to sitting around the campfire, sipping on a beer, smoking a pipe, looking at the stars, telling stories. The agenda is not about getting anything done for work, but just about sharing thoughts. Don't let looking at the web become the focus. If no one is talking, then you are doing it wrong. Meeting once a week should suffice.
Note that all three activities above will go far to introduce new team members to the culture and get them integrated into a productive environment. This is important in the long run as rapid team growth can kill a culture. These activities go far to reinforce hierarchy and retain the culture, as well as identify issues that new members may bring along as baggage.
As a species, we are wired to be social and our social construct is wired to be tribal, both in size and in hierarchy. Also, our tribal roots center on the meal, so adding food to any of the suggestions above is good. Food traditions are the cornerstone of cultures and that is part of our wiring. We learn to trust one another by eating shared food. Food prep is ritualistic, so when possible, rely on each other to make food or acquire food. Making a lunch for all, on site with everyone involved will do more to bond the tribe than anything else, but such a thing may not be possible at your facility. So do what you can to approximate that.