If someone was showing me around or the like, I would walk behind them as a sort of indication of respect because they are assisting me. In the same vein, I would probably prefer, if I was showing someone else around, for them to walk behind me. That's probably a bit unreasonable though.
If I'm walking with people I know, my position relative to their's fluctuates. When I was a little boy, I used to "orbit" groups of people.