OKLAHOMA CITY -- Oklahoma City businessman David Green and his wife, Barbara, are joining a group of some of America's wealthiest families in taking the Giving Pledge. Green, the CEO of Oklahoma-based Hobby Lobby, is in the second group of pledgers taking part in a long-term charitable project launched by Warren Buffett and Bill and Melinda Gates.

The Giving Pledge invites the wealthiest Americans to commit to giving more than half their wealth to philanthropy or charitable causes.

"Like my parents and their parents before them, you don't have to be wealthy to give," David Green wrote in his pledge letter.

"You can give your time, your talents and your passion. We congratulate those who have taken on this wonderful pledge."

Other Oklahomans participating in the effort include George Kaiser and T. Boone Pickens.

The second group of Americans joining The Giving Pledge includes Mark Zuckerberg of Facebook.