It uses your Steemit account. And you don't have to sign up if you don't want to - if you don't buy votes it doesn't matter, and if you buy votes by sending SBD to @minnowbooster you'll get the extra whitelist profit anyway. The only thing you have to have a MB account for is if you want to play the game of inviting and voting on other users. Which is kind of addictive, and can have a nice little residual income to it, but is not in any way necessary.