You can force the usage of a username for the connected user. That mean when a user will connect, even if his username is not the desired one, it will force this group to be used when a file or folder are created. This allows multiple accounts to access the same account but each one with its own password.
In this example, we want to force the user sftp_user to be used by a username user:
<Default> Home /home </Default> <User username> ForceUser sftp_user </User>
The sftp_user user will be used when the user username will create a file or a folder.