They don't have to be, it's just beneficial to choose people who are. Many people might be choosing in advance in case they don't feel like updating their lists every week with people actually competing.
Unless something changed from the beta, you can't have a default team. You just have to go make a new list of competitors each week.
Ahh I can't make a WCA profile. My birthdate doesn't line up
You can create a WCA account without WCA ID, and you just need an account to sign in with Fantasy Cubing, I believe.