Open the main menu of the app and tap "Log out" in the upper right. Try the Sync again and it will ask for your username and password. If your username and password are rejected, go to this page to reset your password.
Can you log into www.setlisthelper.com on your PC or Mac? If not, reset your password here.
Reinstall the app and it should work. You can restore your data with Sync. If that does not work, use the backup.
Open the main menu of the app and tap Sync. Go to www.setlisthelper.com on your PC or Mac. Delete the songs there, then open the app and sync again.
Or in the app, tap the circle next to the song or setlist and choose "Remove".
Open the main menu of the app and tap "Log out" in the upper right. Try the Sync again and it will ask for your username and password.
If the above does not work:
The best way to do it is to put all your PDF files in a Dropbox folder.
This will import your PDF files and create the songs. It will remap the path to the PDF without duplicating the song if there is already a song with the PDF filename attached.
Share the folder with your other user and have them follow the "Update Dropbox Files" steps below:
This feature will look at all the Dropbox files that are attached to songs. It will download the files if they are not there, or update files that need to be updated.