If you are taking part on your own you need to register and create a team that will have just you in it. The team name can be set on this page: https://2020.cipherchallenge.org/my-account/team/
If you want to enter as a group the Team Captain should register first and create a new team. The Team Captain can then send the team members a link to their team from this page: https://2020.cipherchallenge.org/my-account/team/. Team members can then use that link to request to join the team. The Team Captain will receive an email on each request and they can then accept or decline invitations from the team tab on the Team Captain's account page. The team name can be set by the Team Captain editing the name on the Team page under your account.
If you wish to join a team after you have already registered then you will need to change your team. Do this by using the “Change Team” form on this page: https://2020.cipherchallenge.org/my-account/team/. Your new Team Captain will need to accept the invitation.
You can find more information about accounts and teams on the Registration Instructions   and the Joining a Team page.