Joining big Pinterest boards is one of the best ways to get traffic from Pinterest and to get your pins noticed.
Trying to find them from Pinterest itself is a little like looking for a needle in a haystack. However you can easily find Pinterest group boards to join using these two simple methods.
The first is to search for Facebook groups dedicated to Pinterest Board Collaboration. You can also try this on Google Plus but I have not had nearly as much success there as I have had with Facebook groups.
The second way is to use pingroupie.com to find boards on Pinterest - you can then go to the board and find the instructions in the board on how to enter. Usually the board owner will require that you follow the board and either comment on a Pin or send an email to request to join.
Remember that each board that you join will have its own set of rules set by the owner of the board. Pay careful attention to those rules or you may find yourself being removed!
Also keep in mind that while joining big Pinterest group boards you also have the option to add collaborators to your own boards. This is useful since boards with lots of Pins will generate more interest and will rank higher which means you may get much more traffic. In addition to that you control the board which means you have a lot more control.
Here is some other relevant training for Pinterest that you may find helpful:
If you have any questions please let me know and I will try my best to help you!