The Party section in the sidebar has six slots. Each one has its own search bar so you can pick any Pokemon by name or national dex number. The card preview on the left updates instantly as you add or remove.
Step 1
Scroll to the Party section in the sidebar.
Step 2
Click into any slot's search box and type a name or number. Results show with sprites.
Step 3
Click a result to add it to the slot. The card preview updates.
Step 4
To replace a slot, click Clear on it and search again. Leave slots empty if you don't want a full team.