If your groups are Public then directing people to civicrm/mailing/subscribe?reset=1 should show them a form where people can sign up (or unsubscribe) to any of your (public) mailinglists

or perhaps ?page=CiviCRM&q=civicrm/mailing/subscribe&reset=1 for wordpress

If your groups are Public then directing people to civicrm/mailing/subscribe?reset=1 should show them a form where people can sign up (or unsubscribe) to any of your (public) mailinglists

or perhaps civicrm/?page=CiviCRM&q=civicrm/mailing/subscribe&reset=1 for wordpress

If your groups are Public then directing people to civicrm/mailing/subscribe?reset=1 should show them a form where people can sign up (or unsubscribe) to any of your (public) mailinglists

If your groups are Public then directing people to civicrm/mailing/subscribe?reset=1 should show them a form where people can sign up (or unsubscribe) to any of your (public) mailinglists

or perhaps ?page=CiviCRM&q=civicrm/mailing/subscribe&reset=1 for wordpress