Just sign up...also send them an email indicating your interest in playing the weekend and you not having a team. That is what my son did this past year...heard nothing for months and then about 2-4 weeks prior the tryout for the Academic Team was accepted and then were told about a team that the org had put together for orphans like my kid. It was a great experience as a whole for him. Son made the Academic game (that was the biggest deal and goal) and then played the rest of the weekend.
Be prepared as there are a BUNCH of kids that tryout. And many are really good (and supposedly smart) and a few that are not.