A lot of group boards have instructions on them. Just follow those. If there are none, leave a comment on the owner's latest pin, preferably a pin from the same board you're trying to join.
By now, many owners of active group boards are aware of spammers and IM'ers. Don't look like one of them. So don't be obvious by pinning the same images over and over again, don't just pin from your website, etc.
Also, you should start your own group boards and ask owners of other boards to exchange invites. Works like a charm.