Reference article
Recruiting Ducks
Eligible racers can join your flock after you beat them. Accept them from the recruitment queue when a house is ready.
After a race
Winning puts eligible opponents into the recruitment queue. Open it to compare their levels, grades and equipment before accepting them. A duck can wait there while you finish building a house.
Retiring and returning
Retirement makes room in the active flock. Retired ducks can be brought back when you have space and housing, so you can change your team as your needs change.
Making a team
Keep several strong racers rather than relying on one duck: 
Tournaments require three different entrants. Check grades as well as levels, and use 
Stage teaching to help newer recruits catch up.