If I add one guest username 'myguest,' it is added to the config file succesfully. If I open the app again and remove it (make the text field blank) and save it, it is not deleted from the file, and it shows up again in the GUI the next time I run redhat-config-samba.
Also, whatever username is specified to map the guest user to must exist as a user on the local system.
Should be fixed in redhat-config-samba-1.0.1-3. I replaced the GtkEntry with an OptionMenu so that it's impossible for the user to make an invalid choice. Thanks for your report. QA, please verify.
Fix confirmed with 1.0.1-6.