The default value of "Site Members" (viewable by) is missing when adding a message as a site member and the message is only viewable by its creator.
Also, leaving the (erroneous) default value of "Anyone (Guest Role)" does not add view permission for guests.
How to recreate:
Create Organisation "x-org” with site
Add testuser as member of x-org
Create a private page named ”Forum” and add message-boards-portlet to the page.
Log in as testuser and navigate to theportal/group/x-org/forum
Click ”Post new Thread”
Under Permissions, Viewable by, the option ”Site Members” is not available (and thus not the default value).
Even though the value ”Anyone (Guest Role)” is selected, publishing the message and then checking permissions will reveal that only the Owner has permissions set for this message…
This has been tested on a ”vanilla” installation (no custom or unofficial plugins) of Liferay 6.2.1 CE GA2.