I know this has been touched upon in previous posts, but none that I notice had this specific request. Several of my friends do not like to share their activities with the public, so if they want to show me a specific activities, they have to unlock them until I've seen them, then lock them again. If we had a friend list, we could authorize only friends to see our activities, and still keep them from the public. This would also help in finding specific friends activities, mentioned in the other posts.
I would really be interested in this feature too. Several of my friends also have Garmin and use them for kayaking, but right now, there is no good way to share the data. Even making a "friends" list, so that I could quickly find their data would be cool. Right now I can barely find it even though I know it was posted publicly.
Maybe an easy way of implementing it would be to add a third option to the Privacy settings (currently only "Public" and "Private"). Something like "Hidden". In this mode you could share an activity just by sharing the associated permalink. The activities would not be searchable in the Garmin Community, and only people with the permalink could access and view to the activity.
Possibly it should suppose an upgrade of the current permalink format, since it is just build up with the plain activity ID, making permalinks guessable.
