Thanks for reporting this problem! I've fixed the issue, and it will be in the next release. Let me know via private message if you want to give the fix a try prior to the release.
If you are unable to add additional accounts, you can do so manually. Start phpMyAdmin from the Raspberry Pi Programming menu. Username is root and password is 7388. Open the Users table in the station database. Click Copy in the first record and the Insert window opens. Enter the next higher uID and unique Username. Save (click Go at the bottom), and you will have a new account.