To add new members, click on "Add New Group Member", located on the Group Dashboard under "Group Control." (you need to be group owner or be group administrator for this)
Start typing the username (on cachetur.no) to the one you wish to add. Select one of the names that is suggested (make sure it's the correct user, someone could have a similar name). Select member level, and then press the button.
Depending on the notification settings of the user you added, an email notification will be sent.