If you use civicrm-webform, you can add civigroups to your form. You can choose 'user select' to have the user select one or more groups. In the form component, you can choose 'static' to show only a few of all possible groups.
So say you have two newsletter groups and two agegroups. You use the above method to only show the newsletter groups to existing contacts.
Will that remove the the contact from the agegroups if s/he fills out the form ?
Webform-CiviCRM will remove you from a group:
In other words, the user has to make an active choice. Filling out the form as an unknown user, you're not consciously choosing to un check the box (it just defaults that way) so in that instance the module will only add, not remove you from groups.
BTW it works that way for all webform fields. The module will not delete any data from a contact record unless that contact has already been identified and the data was pre-filled on the form and the user made an active choice to erase it.
