If by joining the testnet you mean trying dYdX as a product, all you have to do is go to this site: https://v4.testnet.dydx.exchange
It will ask you to log on with your wallet and sign a msg. Mind you this is on Goerli. Signing doesn’t require gas, so there is no need to get any Goerli’s ETH faucet.
Or did you mean participating in a different capacity?