My standards are pretty high for this. Basically it's just Willow, Xander, Tara, and Anya.
Obviously Willow and Xander are the core. End of discussion. I consider Tara a Scooby because she devoted everything to the cause. Even when she broke up with Willow, she still actively tended to the group. I'm assuming dying doesn't negate membership.
Anya is a Scooby as well. She's helped so much, including providing the central strategy in defeating Glory. She's always there, she's usually helping. She may act aloof and unattached, but all of her actions show otherwise. Basically, if her past contributions don't make her a scoob, then nothing she ever does will.
I don't think Giles is a scoob, because the term is a diminutive one, and Giles is above it all. He isn't on their level. In the Scooby cartoon, there was no Giles. He's something else, something more, or at least different.
Dawn is just there, usually a burden, but very recently she's become of Scoob caliber. I'll wait and see. She has to prove herself a little more because she would be there anyway, amid all the action, because she has nowhere else to go.
Angel and Spike are not Scoobs. They're something else, unless the term Scoob is stretched to levels I'm not familar with.
Oz is not a Scoob, but he was almost one. Basically he was just there for Willow. When things got tough, he bailed. He had a reason to leave, but he wasn't dedicated to the cause as much as the others. In his parting words, he never even mentioned his duties to the group, or anything of that nature. He's in the same boat as Anya, but I think Anya has proven that she cares about the welfare of Sunnydale in a capacity outside of her significant other. I mean, she's being hunted for her life, putting everyone in danger, and she's still in the group, still trying her best and helping people out (it was her advice that got the gang moving in "Get It Done" when Willow was unfocused on getting Buffy back).
As for the others, no way!
I would describe a Scoob as a member of the supporting group underneath Buffy, dedicated to the same cause as her, and who in general provide friendship and/or support. They actively safeguard the welfare of Sunnydale in a manner complimentary to Buffy's role as a Slayer.
Sound good?