What do dozens of American billionaires talk about when they get together? Their topic this week was of course money; not how to make it, but how to give it away.

Billionaire investor Warren Buffett said Friday that a private gathering was a great chance for the billionaires who have pledged to give away at least half their wealth to meet each other, compare notes, eat and laugh.

The media was banned from Thursday's first meeting of the group that has accepted the giving challenge by Buffett and his friend Microsoft co-founder Bill Gates. Since last June, 69 individuals or couples have made the giving pledge.

Buffett knew only about 12 of the 61 people at the dinner at the Miraval Resort in Tucson before the famously gregarious Berkshire Hathaway CEO worked the room and made 40 new friends.

"They all more than fulfilled my expectations," Buffett told The Associated Press in a telephone interview.

Melinda Gates, co-chair of the Bill & Melinda Gates Foundation, said she was delighted by the openness of the virtual strangers. At one point, conversation at her table drifted toward the biggest mistakes people had ever made as philanthropists.

George Kaiser, a Tulsa, Okla., philanthropist who aids early childhood education and social services programs, said the giving pledge helps philanthropists who don't want to just throw money at causes and instead want to explore the best ways to invest money to tackle the world's biggest problems.

"Being able to share with other people who are agonizing about the same decisions is extraordinarily useful," said Kaiser, the chairman of BOK Financial Corp who has been an oil and gas industry executive for four decades. He led a session on applying analytical business practices to philanthropy.

The goals of the organization do not include working together to pool philanthropic dollars. Still, the meeting in Tucson that ended Friday included sessions where different philanthropists shared their passion to improve education, the environment and other causes.

Some common themes emerged from the event. The participants are looking to do more impactful, more effective philanthropy and to inspire average people to give money away, Jean Case said.

Sharing ideas about giving also took place informally. Melinda Gates said she talked to two people who were devoting money for work on state pension issues and criminal justice -- problems Gates had previously not thought about.

Chuck Feeney, a New Jersey philanthropist Buffett called the spiritual leader of the group, spoke about his plans to give all his money to charity.

"He wants his last check to bounce," Buffett said.