The goals for the twice-yearly meetings, the brochure says, include attracting more investors to the cause, but also building institutions “to identify, educate and mobilize citizens” and “fashioning the message and building the education channels to re-establish widespread belief in the benefits of a free and prosperous society.”

Charles Koch, whose wealth Forbes magazine calculates at about $21.5 billion, argues in his letter that “prosperity is under attack by the current administration and many of our elected officials.” He repeatedly warns about the “internal assault” and “unrelenting attacks” on freedom and prosperity. A brochure with the invitation underscores that to the Koch network, “freedom” means freedom from taxes and government regulation. Mr. Koch warns of policies that “threaten to erode our economic freedom and transfer vast sums of money to the state.”

The Kochs insist on strict confidentiality surrounding the California meetings, which are entitled “Understanding and Addressing Threats to American Free Enterprise and Prosperity.” The letter advises participants that it is closed to the public, including the news media, and admonishes them not to post updates or information about the meeting on the Web, blogs, social media or traditional media, and to “be mindful of the security and confidentiality of your meeting notes and materials.”

Invited participants are told they must wear nametags for all meeting functions. And, ensuring that no one tries to gain access by posing as a participant, the invitation says that reservations will be handled through Koch Industries’ office in Washington: “Please do not contact the Rancho Las Palmas directly to place a reservation.”

To give prospective participants a sense of what to expect, Mr. Koch’s letter enclosed a brochure from the group’s meeting at the St. Regis Resort in Aspen, including a list of the roughly 200 participants  a confab of hedge fund executives, Republican donors, free-market evangelists and prominent members of the New York social circuit.

Image Koch Industries, an energy and manufacturing conglomerate run by the billionaire brothers Charles, left, and David Koch operates a foundation that finances political advocacy groups, but is protected from having to disclose much about what they do. Credit... Left, Mike Burley/Topeka Capital-Journal via Associated Press; right, Robert Caplin for The New York Times

They listened to a presentations on “microtargeting” to identify like-minded voters, as well as a discussion about voter mobilization featuring Tim Phillips of Americans for Prosperity, the political action group founded by the Kochs in 2004, which campaigned against the health care legislation passed in March and is helping Tea Party groups set up get-out-the-vote operations.