I'd have to say that Buffy, Willow, and Xander are the original Scoobies, that's a given. Then Cordelia kept getting pushed into the mix and decided to help out, before AND after Xander. Oz also got pushed becasue of Willow, but he definately still did work. I mean, not all boyfriends do EVERYTHING with their girlfriends, so I think of Oz as a Scooby. Anya, just like Cordelia, wasn't a Scooby at first. She was just Xander's girlfriend. She didn't go to all of the Scooby meetings. but by season 5, she became an official member. The same is true for Tara. She wasnt vital at first, but became an official Scooby during Season 5.
As for Spike, Angel, and Riley, they were in and out, mostly just doing their own thing. They weren't part of the core group. Dawn was pushed in becasue she's Buffy's sister. Although she has started to do more and more research, she isnt a core member. Neither is Giles. even though Giles is alwasy around, he's more of the father figure. When I think of the Scoobs, I think of the group of friends that do research AND hang out together when the world isn't in peril.