It always bugged me that JKR insisted that being in Slytherin didn't make you "bad", yet hardly ever showed examples of Slytherin students being "good".

But I do understand why there weren't any there. The house, in general, just has too much pride for that. I don't think any self-respecting Slytherin would want to be caught dead in a club called "Dumbledore's Army" run by the boy who lived, a blood traitor and a mudblood. The other Slytherins would make their life hell when they found out.

It would have been nice indeed but not very realistic, as Mongoose explained. Besides, the Inquisitorial Squad had a strong appeal to Slytherins - gaining extra points, etc. But yes, having some Slytherins that could have managed to hide their membership to the DA quite well, would have been interesting. A real challenge for their cunning. After all, there was Severus in the Order and Horace was one of these Slytherins that were not prejudiced.

Not all Slytherins have to be evil; Slytherins are about ambition, not selfishness. Sometimes, their ambitions require them to be selfish, that's why they can be really mean. I really support House Unity, and it would be really great if Jo Rowling made in least 1 or 2 Slytherins in Dumbledore's Army. It was too bad that all the Slytherins in the book were evil (for the ones that went to Hogwarts at Harry's time).

Has anyone seen good Slytherins? Not saying all are bad I just wondering if any are good?

Snape, Slughorn (even if he can be a bit of a prat), Regulus (once he saw the errors of his ways). There are probably others, I just can think of any at the moment.

Never.

I couldn't see any of the pre-existing DA members trusting them enough to tell them or invite them. And a lot of Slytherins would see telling Umbridge as serving their ambition.

My thoughts on the subject is their ambition to prove they are not evil would prompt one or two Slytherin's to join. I feel it would be also something they would take pride in.

I like the idea in theory of a handful of Slytherins rebelling by joining the DA, in the same way you have Slughorn and Snape defying Voldemort. But in practice it wouldn't have worked.