To do this you don't need to use the permission system, actually. What you would do is to create line.tpl in the normal way, but in the full.tpl you would do a check on the users group.
You would check if a user is member of a certain group. If the user was part of that group you would show the content, if now you would show a text that the user needs to log in before the content is shown. This can be done with section code in the template.