A few days ago, shopping in a local supermarket, I noticed a stranger and promptly categorized him as one of us. He was (I'm guessing) in his twenties, somewhat overweight, wearing shorts, a T-shirt, a scruffy beard, engaged in animated conversation with two younger companions. Animated conversation aside, none of that describes me. My instinctive reaction reflected the fact that he fit the pattern of people in environments where I am comfortable. He was probably an sf fan, probably a board game or computer game player, possibly a World of Warcraft player, possibly an SCA member.
Like most moderns, I am a member of more than one tribe. Some years back, when we were visiting colleges that one or the other of our children was considering, I took advantage of that fact to find sources of information not funneled through the admissions department. Part was locating members of the local SCA group, if there was one, and talking with them about the school. Part was wandering around the economics department getting into conversations. Economists are more willing to talk freely to a fellow economist than to a random parent, a trait I took advantage of.
For one final and stranger example, I offer my response to seeing someone else driving the same model and color of car I drive. I know nothing else about the driver, but my instinctive reaction is to categorize him as a sort of distant kin.