How I do to participate of a group or campaign?

To your left is the main menu, click on Campaigns. In there you will see a list of all active campaigns and ones awaiting approval. Click on one you are interested in and hit the join button. Your name will be added to the list...as long as its an active campaign. Hope that helped.

Of course you helped me, thank you very much John..

Cheers

Besides enlisting, you also need to read, and understand the campaign parameters, as to start dates, ending dates, and what acceptable, and what is not acceptable.
Usually a PM to the campaign leader will answer all your questions, about each campaign you decide to enter.