You don't have to sign up! Just grab the files and play. Of course, if you'd like to notify the rest of us that you are playing, you may do so here in the Info thread. Don't post any info about your results, though, until closing day (May 12). Of course, if you have a technical problem, or questions, you should post those here, too.
There will not ever be any signups required for any Imperiums, with the possible exception of some sort of scenario where players would receive different start files for some reason. Lacking an editor to modify maps and scenario information, right now that seems unlikely.