Once you've added players to your team and you've selected a seat for each one of them, you're ready to start a Team Match.
Click on the "Ready for Match" button on the top right corner. This will prompt an invitation to Join a Team Match to your team members.
Once they all accept the invitation, your team will be marked as Ready to Play and you'll see a dialog with other Ready to Play teams.
Select an opponents team by clicking on the "Match Now" button under the team's name (a team of bots is always available to play)
Once the opponents team accepts your invitation to play, you'll be shown the quick team match settings.
Finally, click on "Start Match" to start the game!