How can I allow opt-in subscription to specific groups with Webform CiviCRM?
Once you have selected 'user select' for the groups in the CiviCRM section of the webform admin section, save and return to the 'components' tab (where all the webform fields are listed).  Find your Groups field and edit it from the component list. If you change the list from Live to Static, you can then select which of your groups will appear as an option to the user.