When you join a new Team, you will be asked if you'd like to make it your "Default Account". This means that all future deliveries will be automatically booked under that account.
You can, however, change this at any time.
To change your "Default Account":
Go to the "Team Management" screen from the main menu:
Select the Team you wish to edit / set up from the Membership selector
If the Team is already set as your Default Account, it will show a "Default" tag next to the account name. Otherwise, you can set it as your default by clicking "Set as your default".