I don't think Spark is dead, or at least I hope it isn't. Otherwise I'm wasting my time maintaining it, fixing bugs, replying to people's requests for help and generally trying to increase its visibility without any help from Microsoft.
Sorry if you haven't had any replies from devs, but I'm not sure who those devs are - I certainly haven't ever received an email from you and I'm the primary developer on the project so I have to assume you didn't try very hard I'm afraid. Joining the group shouldn't be an issue - I just tried it with a different email account and was instantly on, so I'm not sure what the problem is there - sorry.
Spark is fairly mature as a product which means it's documentation is up to date, and most people don't need much. There are a few things in the development pipeline, but nothing urgent, so there isn't much activity on the list. Just today however, I replied to another email so it's still going for sure and any help you'll need will be provided I'm sure.
Just recently, Spark support has been added to both FubuMVC and NancyFx frameworks so I'd say there's life in this old gal yet.
Hope that helps.