The term groupie used to mean that chic who blew all the band guys on the back of the bus, but I think these days it covers a much broader spectrum.
To me a "groupie" is more along the lines of what Squealie said. I see what you're saying about "fans", but there are fans, and there are FANS. I'm a fan of many groups, but I ain't centering my free time around them. A groupie is that person (about 99% of the time a chic, but not always) who goes way out of their way to support the band. Its more about that person wanting to genuinely be a part of the band to an extent, and being willing to do whatever to help out.
...and occasionally theres a pole-smokin involved.