Redwood City, Calif.

Three people who have  signed the Giving Pledge—a commitment to give away at least half of their fortunes—spoke on Thursday about why they think rich people should donate more and what they hope the pledge will achieve.

John Morgridge, chairman emeritus of Cisco, and his wife, Tashia, were joined by Lorry Lokey, founder of Business Wire, for an evening conversation at the Global Philanthropy Forum, an annual meeting for donors.

The Morgridges were among a small group who attended the very first dinner organized by Warren Buffett and Bill and Melinda Gates in May 2009, even before the pledge idea was fully baked. Mr. Morgridge said that people at the initial dinner floated the idea of doing videos and interviews to promote the pledge to others but instead decided that hosting more dinners would be the best way to try to recruit other wealthy people to join.

After that first dinner, the Morgridges said they took a little time to think over whether they wanted to sign the pledge. Mr. Lokey said he agreed while on the phone with Mr. Buffett, who called him with the proposal.

“I said yes right away, because the money was already gone,” joked Mr. Lokey, who has already given most of his fortune to education, medical research, and other causes. “When you pitch in and make it public, it encourages others.”

He continued: “We’ve got all kinds of billionaires sitting on their rear ends and doing nothing. [Mr. Buffett] said he wanted me to be an example of how far beyond 50 percent you can go.”

Mr. Morgridge said he also felt that many rich people were spending too much of their money on extravagances and could afford to donate much more. He and his wife started the Tosa Foundation, in Portola Valley, Calif., and are big supporters of education and other causes.

“This valley has a lot of wealth, and, for my taste, a disproportionate amount of it is spent on homes that they live in for two weeks of a year,” he said. “We didn’t create all this wealth, we capitalized on an environment that permitted us to create it.”

But persuading other people to publicly commit to give more of their money away hasn’t been all that easy, the donors said.

Mr. Lokey said he was uncomfortable asking other people to donate. “I’d make a lousy development director.”

The Morgridges hosted the third Giving Pledge dinner at their house, which they said resulted in only one additional person signing the agreement.

Many people seemed to be held back by concerns about how to treat their children in their financial planning, the Morgridges said. Others just didn’t seem to be thinking much about a long-term approach to giving. Said Mr. Morgridge: “I was a little surprised at both dinners that there were a fair number of people who really had no philanthropic plan.”

Another challenge has been convincing those people who give but who do so anonymously. The Morgridges started out in that camp, said Ms. Morgridge. Their first large gifts were anonymous. But, as time went on, “people found out about it anyway,” she said. “We finally realized it was kind of a sham that we were anonymous.”

Asked about concerns surrounding the undemocratic nature of mega philanthropy, Mr. Morgridge pointed out that people use money to buy influence in all realms, not just through their charitable giving.

“You’re not worried about the lobbyists in Washington, D.C., who influence how government money is spent? You’re not worried about the money that’s being spent to influence politics in this country?” he said. “To say we’re less trustworthy than the government or the lobbyists or the corporations—I don’t think you can prove that case.”

All three donors emphasized how much they enjoy giving.

“You can give $30-million for student aid and every spring the letters roll in thanking you,” said Mr. Lokey. “It’s a wonderful life.”