Since most Pagan circles don’t have specific buildings associated with them, it’s kind of hard to just drop by on Sunday morning, as it were. If you happen to stumble across an outdoor circle, the main thing is don’t just stare from a distance - that makes people feel VERY uncomfortable. If you actually are interested and you want to find out what’s up, you can wait (discreetly!) until the end of the circle and talk to the priest/priestess.
Of course, if a friend invites you to a circle, you should rely on him or her for pointers. Every circle is different.
Is this the same Bob that was found smoking a pipe in the basement of rental houses? If so, you may have a behavior disorder demon on your hands. Better get him under control before he begins harp lessons!