Hover over Competitions and select Manage Competitions.
Select your competition.
From the Teams tab, click Add Teams.
NOTE: you will see a list of any teams already in the competition.
NOTE: before adding a team, make sure the competition has spare places for a new teams. Each competition is set up with a limit to the number of teams allowed. You cannot add more teams than what has been specified.
Tick the box next to the teams you want to add. Click Add Teams. The teams will move to the right side.