When, as a member of a private (invite-only) group, I open a forum discussion the text of that discussion's topic is visible on my member profile page to anyone. The forum topic shows up at the bottom of my profile page under the heading of "Group Forum" and is hyperlinked to the discussion page.

On the plus side, diigo correctly blocks access to the discussion (i.e., clicking the link will result in the display of an error page if you aren't a member of the group) but even revealing the topic to the public is wrong. Imagine, for example, that I have formed a private group related to marketing and product research for a business. It is likely that even just the titles of the discussions will reveal "too much information" to any "outsider" interested in what the business is researching or developing.

Information from invite-only groups should NOT be made public.