CiviCRM Community Forums (archive)

Interesting requirement 

First question to answer is where this data would be stored in the CiviCRM data model. One place would be as custom data linked to the "members" contact record. BUT this would require implementation of our planned (but not yet in place) feature to allow multiple rows of custom data values for a given entity. (Each public person's comment would be an instance of the custom field value.)

Maybe easier option is to use the Activity record - which would allow you to store a foreign key to the contact making the comment (source_contact_id) and the contact being commented on (target_contact_id) - as well as the comment ("details" column). So far so good, but there is no out of the box way to do this. Probably best bet would be to create a separate module which puts up a form for comment, and then uses the APIs to insert the activity and activity_target records. You could then customize your Profile view page to add a link to this form.