If you want to be competitive, register in the Elite wave. Otherwise, you can register in any wave you want and your schedule. Once a wave is full, you can no longer register in it, you must choose another one. The system will tell you when you register.
You must register early if you are a team. Once a wave is FULL, other runners from your team who are not registered will not be able to run on that wave (but they can still be part of your team!)