A furpile is a group of furries who are at least partially one on top of the other, possibly in fursuits. It's usually more of the result of "everyone trying to get comfortable" rather than a deliberately planned thing. A furpile is usually not sexual, although the director of CSI decided it would be more fun to depict one that was. Another example where furpile was represented sexually in the media was the movie Crazy/Beautiful when the heroine's best friend said "...the term 'furpile' came up a few times" in reference to a past conversation she had, planning a party. If one watches the movie, one is led to believe that all parties went to by the twosome involve sexual activity.
It's a good idea not to pile up on others if one's weight is over a certain limit. When those on the bottom show signs of discomfort, it is best to let them out.