Group Boards on Pinterest are very difficult to join, you must receive a special invitation which is difficult to get.
They are also a very good way of marketing your content or products. What you post in group boards will appear on the homepage of every single user in this board, thus giving your post the chance to be viewed (and also liked and saved) by a maximum of people. I is also a very efficient way to make a pin go viral and be saved thousands of times.
My username is TheJGfiles, check out my profile, choose the group boards you want to be invited to and send me the list.