The current GB (D-Day Stripes) ends on May 31st, so you might still have time. You'll find the rules, and information on photo requirements, posted in the Group Builds section.
Basically, each GB lasts four months, with a one month overlap between each build. For example, the current build ends on May 31st, but the next GB (Cold War) commences on May 1st, ending on August 31st, and so on.
Terry.