If you only seem to be able to add 1 person or team to your competition, you may have selected a "goal tracker" competition type.
The goal tracker competition is designed for one user or team to work toward a specific goal.
Which competition designs should I choose for multiple players/teams?
Multiple players and teams listed on a leaderboard
Your top three performers will share the spotlight on this leaderboard
Head to Head
Pit 2 players/teams against one another
Multiple players and teams can compete against each other in a race or an elimination-style competition.