[heymatty]

I would go where Grooveshark users are. Find who they are and reach to them. Use twitter search and other social monitoring tools to find them. http://lithium.com has got a good social monitoring tool you can use free for 14 days. Should be plenty of time for you to find enough of them.

Once you found them, ask them if they would part take into grapev.in mavens. Tell them you are looking for feedback from heavy grooveshark users and in return they will be the only ones with early access to some grapev.in features.

Create them a special account, give them a place where they can connect to each other. Make them super fans by iterating and get to market fit with them.

They will evangelize your product for you - sounds like a good starting point.