You can join a team whether or not you have bought your entry yet.
If you have not entered yet
At checkout, start typing the team name in the Select Team field and pick your team from the list. If the Team Captain set a team password, you will need it.
If you have already entered as an individual
Make sure you are logged in, then go to My Entries on your Dashboard.
Find your ticket and select Edit Entry from the Entry Actions menu next to it.
Find the team you want to join.
Select UPDATE ENTRY.
You do not need to cancel or rebuy your entry to join a team.
If you cannot find the team, check the spelling with your Team Captain. Team names have to match exactly, and a team created for a different event will not appear.