If you've signed up as an individual but convince friends to join you in the Mud down the track, don't stress as it's still certainly possible to create or join a team after registering. This is done through your My Events Portal.
A link to this portal can be found at the bottom of your confirmation email and should allow you to log in right away. Alternatively, you can click on this link which will take you to the same page: https://myevents.active.com/ToughMudderAustralia
Once logged into the portal, click on the button that says 'Create a team' if the team hasn't yet been created, or join team if someone has already created one for you. (See attachment for reference)
You can also send out invitations for friends and family to join your team through this portal if you are the team creator (team captain).