From GroupScript documentation
You can export the users table from the old database, but be sure to set these settings when exporting from phpMyAdmin:
- Check foreign keys = 0
- Structure = 0
- Data = 1
This data should import cleanly.
Also, you will have to replace the encryption key to the old one, if you will import the users from previous database - it is located in system/application/config/config_local.php Otherwise your users will not be able to log in.