"After a few forty, fifty, sixty hour weeks, inevitably you're going to create a friendship with some of the people that you work with. And here's how you handle it.
You need to have boundaries. You need to have a conversation before your relationship intensifies, and gets heated, and you may have a personal disagreement. You want to talk about what the boundaries are. So this is the stuff that we'll talk about at work; this is the stuff we're going to talk about in our personal lives. And when you have a friendship at work, it's really important to start to divide that personal time and that professional time, both for the sake of your friendship and for the sake of your professional relationship. You've got to have some boundaries, and you want to discuss those boundaries before the fact.
You also want to be really careful not to make other people feel alienated, as if they're not a part of your clique. You want as much as possible to keep those kind of intimate, friendly kinds of excursions and conversations out of the office, so that people don't feel uncomfortable. You've got to keep these things in mind, so that you're not excluding the potential for new relationships in the workplace.