Seems to me that you don't need to sign people up to play unless they want to be the Poster. Anyone else should just be able to jump in and guess the mystery word.

I'm sure that's true. As I understood it, maverick simply wanted to make sure that there would be enough participants to make it interesting. Surely, anyone who guessed the word would jump at the chance to set their own mystery word? In the unlikely event that my meagre wits stumbled upon the answer, I would love to set the next one, something discreet, like supercallifragilisticexpialidocious, for example.