I'm not sure it will allow an easy way to control/limit the number of individuals added, but it might be possible to do this via a View that lists people that are linked to the contact person's Org and provides customized links from the view to a webform (passing contact IDs via URL to populate the webform). You may also need the Relationship Permissions ACL extension and have these relationships set up to grant editing permissions.

I suppose if the benefit is to allow up to 5 members listed, you could just say that even if they add more than 5, only the first 5 will be selected to be listed in the directory.

It might be possible to do this via a View that lists people that are linked to the contact person's Org and provides customized links from the view to a webform (passing contact IDs via URL to populate the webform). You may also need the Relationship Permissions ACL extension and have these relationships set up to grant editing permissions.

e.g. If you have a view that is pulling the current user (contact person with permissions to add/edit), you can use the View to also get the Org and the other members based on relationships, then feed them into a Webform with CiviCRM functionality enabled, feeding the contact IDs into the webform using the URL. ("If the url contains e.g. cid1=123, it will be used to pre-populate this contact (takes precidence over other default values).")

You could set up the form to only allow input for 6 contacts (the org and 5 members) and specify their relationships in the webform/civicrm settings, you can feed the contact IDs if they exist, or allow new contacts to be created if they don't.