To add an availability database to this availability group, you will need to: Select the new user database on this instance ...

To add an availability database to this availability group, you will need to:
•   Select the new user database on this instance of SQL Server.
•   Select your initial data synchronization preference.
•   Optionally, connect to all the existing secondary replica in the availability group.
•   Check the validation results of adding the new database to the availability group.
•   Review your selections.