Forty wealthy families and individuals have joined Microsoft Corp. co-founder Bill Gates and billionaire investor Warren Buffett in a pledge to give at least half their wealth to charity.Six weeks after launching a campaign to get other billionaires to donate most of their fortunes, the chairman and CEO of Berkshire Hathaway Inc. released the first list Wednesday of people who have signed what he and Gates call the "giving pledge."Buffett said he, Bill and Melinda Gates, and a few others have made 70 to 80 calls to some of the wealthiest U.S. individuals. The people who agreed to the pledge are from 13 states, with the most participants in California and New York.Participants include Mayor of New York Michael Bloomberg, filmmaker George Lucas, media mogul Ted Turner and Microsoft co-founder Paul Allen.Buffett decided in 2006 to give 99 percent of his fortune to charity. Then, he was worth about $44 billion. After five years of investment returns while making annual gifts to five foundations, Buffett's fortune totals nearly $46 billion.