If the rule is given as a condition of attendance, then it could reasonably be considered a contractual obligation. The event organizer could then sue for damages on the basis of breach of contact. The extent of those damages would be related to how much reputational damage the venue or event organizer suffered, and the potential loss of future attendance caused by that.
In addition, both the subject of a secret conversation and the participants of that conversation could sue someone for disclosing the discussion on the basis of libel. My understanding of USA law is that libel has a very high threshold and is therefore rarely litigated, but in other jurisdictions, such as the UK, libel can be as simple as saying something true with the intent to hurt the subject's reputation.
Many people try to put as much as possible into bespoke, written contracts, but usually a mixture of common law and implicit contracts is adequate to litigate almost anything considered harmful by society. I doubt the NDA is actually needed as long as the Chatham House rule is made clear.
In addition, both the subject of a secret conversation and the participants of that conversation could sue someone for disclosing the discussion on the basis of libel. My understanding of USA law is that libel has a very high threshold and is therefore rarely litigated, but in other jurisdictions, such as the UK, libel can be as simple as saying something true with the intent to hurt the subject's reputation.
Many people try to put as much as possible into bespoke, written contracts, but usually a mixture of common law and implicit contracts is adequate to litigate almost anything considered harmful by society. I doubt the NDA is actually needed as long as the Chatham House rule is made clear.