First you have to create a match. In the match feed you can create players while:
Adding todays Team
After the match while Voting MVP
You can add players by pressing [ + add players ] button.
The created players will show up in every player overview once you’ve created them. This way you can already gather stats for your whole team.
Note: Currently on Android it’s only possible to create teammates on future matches. Matches that are finished do not have this functionality yet. If you create your teammates in a future match they will be available in finished matches also.