You'll need help of group boards. To join them, you'll have to contact board owners. What worked for me is asking them nicely to add them to their board. Of course, you should have nice, non spammy profile. Just comment on one of their latest pins, tell them you like their "name" board and you would like to contribute there. The more you ask, more invites you'll get.
Do not use automation for this, make these comments manually and make sure they look human