You don't need a Gridcoin testnet wallet nor do you need to pay anything to play - this is the peerplays testnet & you're given 100 tokens to test the system with.

In order to play, you need to sign up to the testnet website yes.

I'll add more links to the Gridcoin website.

Edit: I've added a description about Gridcoin, and there are instructions for users to post both their GRC (production) address in this thread for rewards, and their Peerplays testnet username (to prove they participated).