Contribute to group boards that have a large following. Make sure that your pins are relevant to the topic of the group board and follow the rules that the administrator has supplied. Group boards are by invite only- so you will need to research and see if the boards are accepting contributors.
Admins will tell you how you can request an invite: some will want you to tag them in a comment on one of their pins, some want you to email them and others want you to go to their website contact form. You must be following them and the board that you wish to receive an invite.