I'm not sure what you are doing wrong, but I can suggest a way to debug.
Run the adduser script using bash debug features.
# bash -x adduser
This will display each line of the script as it executes. After you have entered all the required info. Watch for the call to the useradd program. After the script has errored out, type in the useradd command exactly as the script tried to execute it.
Now you can modify the useradd command line until the user gets added. Take the successful parameter sequence and/or format and modify the script write the useradd command that way.
If you really feel that useradd will not take the group command as it should, then download the useradd source and debug it.