The easiest way to set the "group" or any other submission parameter to always display a specified default value is to press the "Set Defaults" button at the bottom left of the submission dialog. This will store the values set in that submission dialog for that jobtype and user to always come up by default.
The alternative is to modify the mayabatch.py submission dialog directly and force the default value to a specified group. For example, under the cmdjob.package['-renderer'] line around line 1297, add:
cmdjob.properties.setdefault('groups', 'mygroup')