Their commitment is to give away at least 50% of their wealth in their lifetimes or at death to charity. Buffett and the Gateses developed the idea believing that a quantified goal would help the wealthy to think through their philanthropic plans. (For the full story on the $600 billion challenge, click here.) “Obviously there are a host of people we can recruit, and certainly we’ll be signing up many more,” says Buffett. “But I will tell you, I would have thought the Giving Pledge a success at levels well below 81.”

Encouragement of a personal kind is often important in convincing a member of the superrich to make this big commitment. Last fall, Buffett hosted a private dinner at a Southern California restaurant to talk philanthropy with some of his fellow billionaires. The event inspired Henry and Susan Samueli, who have a long history of charitable giving, to make the Pledge. Henry Samueli cofounded semiconductor giant Broadcom (BRCM) and, today, is its chief technical officer. “We want to see how much good we can do in this world while we are alive,” says Susan Samueli. “There is no reason to save it until after we are dead.”

Tech wealth figures prominently among the new members of the pledge list. In addition to the Samuelis, new signatories include Elon Musk, a serial entrepreneur who is chief executive of Tesla Motors; Michael Moritz, the venture capitalist who helped to finance Google (GOOG), Yahoo (YHOO) and PayPal (EBAY), and his wife the novelist Harriet Heyman; John Sall, who co-founded software maker SAS Institute, and his wife Ginger; and John A. Sobrato, a prominent Silicon Valley real estate developer, his wife, Susan, and their son, John Michael Sobrato.

Other new members include two hedge fund proprietors, activist investor Bill Ackman, of Pershing Square, signing with his wife, Karen, and Glenn Dubin, of Highbridge Capital Management, and his wife, Eve; Arthur Blank, cofounder of Home Depot (HD); Steve Bing, of Los Angeles, a founder of Shangri-La business group; Edgar Bronfman, of the Seagram’s empire; Red McCombs, of the San Antonio automotive fortune; and Ted Stanley, founder of MBI, a developer and marketer of fine collectibles, and his wife, Vada.

None of the signatories is new to philanthropy.

Musk, who is also a co-founder of PayPal as well as founder and CEO of SpaceX, which makes rockets and other space launch vehicles, says he had already committed in his will to give away most of his fortune to charity. He was persuaded to join the Giving Pledge after his friend Jeff Skoll, an eBay co-founder and fellow pledge signer, convinced him it would give his philanthropy more impact. “This is not a change in what I am doing,” Elon Musk says. “It is publicizing what I am doing in hopes that others will follow through.”

Musk, 40, has established the Musk Foundation, which helps to promote science education, pediatric health and clean energy. “To some degree, the reason I’m doing the companies that I am doing is to have a positive effect on the world,” says Musk, who is also chairman of SolarCity, which makes solar energy systems for homes and businesses. “Donating my assets to charitable organizations upon my death will probably lend some credibility to what I am saying about the motivation for [my charities].”

Musk says his desire to become involved in philanthropy was sparked, in part, by his upbringing in South Africa, where he witnessed a lot of hardship. “If I could do something to alleviate it, I wanted to do that,” he says. “There are problems not taken care by the government or the market. That’s probably the best use of my assets.”

Similarly, Henry Samueli says joining the pledge won’t alter the couple’s philanthropic efforts. Through the Samueli Foundation, they have focused primarily on three areas: science, math and engineering education; integrative medicine, which combines Western medicine with complementary or alternative treatments; and promoting Jewish culture. “You wouldn’t join the pledge if you weren’t already committed to giving,” Samueli says. “It is really all about sharing a vision, sharing ideas and sharing best practices with like-minded individuals.”

The couple says that their children, whose ages range from the late-teens to the mid-twenties, are supportive of their philanthropic endeavors.

Another supporter of Jewish causes is Edgar M. Bronfman, 83, who contributes heavily to the welfare of Jewish youth, mostly in the US but to a degree in Israel. Bronfman says he was encouraged by his friend, retired investment banker Pete Peterson, an original signer of the Giving Pledge, to join the roster. In the letter Bronfman is posting on the givingpledge.org website, where most signers express their thoughts about philanthropy, Bronfman says, “I have found philanthropy deeply satisfying work…a joyful experience.”

Pershing Square managing partner Bill Ackman got his push from Warren Buffett, who called Ackman to urge him to sign on. The idea fit Ackman’s charitable notions beautifully, but he has three young daughters and didn’t much like the idea of their googling their parents and discovering how rich they were. Buffett in effect said, “That train has already left the station,” arguing that major wealth is really something you can’t hide — and certainly not if you want to see it have important philanthropic effects.

Ackman’s biggest charitable interest is what he calls “social entrepreneurship.” For example, he has given millions to back the efforts of Andrew Yuon in Kenya to raise the ability of the country’s so-called “one-acre farms” to provide an adequate living to the families tilling this land.

In the letter Bill Ackman is posting on the Giving Pledge website, he expresses a thought not too common in the other letters there: “While my motivations for giving are not driven by a profit motive, I am quite sure that I have earned financial returns from giving money away. Not directly by any means, but rather as a result of giving money away. A number of my closest friends, partners, and advisors I met through charitable giving. Their advice, judgment, and partnership have been invaluable in my business and in my life. Life becomes richer, the more one gives away.”