Assign a Default Member Group for e-Newsletter

Our website (http://deplomp.frankswebsites.nl/) will only have a single member group. Is it possible to uncheck the 'Show Groups' checkbox for the 'eNewsletter Subscribe' widget (so that member groups are not displayed), and when the user clicks the 'Subscribe' button, the single (default) member group is assigned to the newly subscribed user? At the moment, with 'Show Groups' unchecked, the user is not assigned any groups when they click the 'Subscribe' button...