Hi Yunus,
Go to that page, choose the last seen date (e.g. the day of the last monthly mailing) and the username of a regular user who does not have access to private content. Then you will see the summary email containing all the new content you want to share.
How you get it from here and out via email is up to your preference. What I do is, using chrome, select the text I want, then right click on it, then select INSPECT. From the inspector pane I then copy the HTML of the text I want into the clipboard and then paste it into the source HTML part of a mailchimp content block. From there I make some minor tweaks, add an intro “ask”, and schedule the mailing! 
(note: mailchimp is a bit fussy about formatting so be sure to avoid editing your discourse HTML in the wysiwyg.)