From Marianist Province of U.S., Inc. v. Century Indem. Co., 2010 U.S. Dist. LEXIS 110888 (D. Colo. Oct. 5, 2010):

In Colorado, the joint defense (or common interest) doctrine is an exception to the general rule that the attorney-client privilege is waived when privileged information is disclosed to third parties. Black v. Southwestern Water Conservation Dist., 74 P.3d 462, 469 (Colo. App. 2003)***. "Communications shared with third persons who have a common legal interest with respect to the subject matter thereof will be deemed neither a breach nor a waiver of the confidentiality surrounding the attorney-client relationship." *** The common interest doctrine includes pre-existing confidential communications and documents that are shared during a common enterprise. Id.; see also Ritter v. Jones, 207 P.3d 954, 960 (Colo. App. 2009). "The privilege applies only to communications given in confidence and intended and reasonably believed to be part of an on-going and joint effort to set up a common legal strategy." Black, 74 P.3d at 469 (citations omitted). ***

Brother Stephen Glodek of The Marianists *** testified that while The Marianists and the diocese (Bishop of Pueblo) operated under a joint defense agreement, he "supposed" that [their respective counsel] Burg Simpson and Tim Schimberg exchanged "correspondence regarding draft but not finalized joint defense agreements between 2005 and the middle of 2006," and that a "sticking point" in the negotiations was "a complication with North River Insurance Company being a part of this case and the diocese's relationship with North River." .... In response to the next question, "So there would have been no agreement then in place between the diocese and the Marianists in 2005?" Brother Glodek answered, "I don't think so, no. I forget the date that all that was agreed upon." ***

Defendant contends that Brother Glodeck's testimony proves there was no agreement and no meeting of the minds necessary to invoke the joint defense privilege in 2005. The Court disagrees. The deposition questioning clearly focused on the existence of a written agreement and Brother Glodek provided answers reflecting his understanding of whether and when a written joint defense agreement existed. As stated above, the privilege applies to communications given in confidence and intended and reasonably believed to be part of an ongoing and joint effort to set up a common legal strategy; written agreements are not necessarily required. See Black, 74 P.3d at 469. Here, Mr. Laman attests that "[b]y their oral agreement and subsequent conduct, as well as a tacit understanding throughout the course of the litigation that all such discussions and communications between the parties and their counsel were subject to the said joint defense agreement, the parties operated at all times as if the attorney-client privilege was in full force and effect." *** The emails at issue here occurred during the litigation involving co-defendants The Marianists and the Bishop of Pueblo, were exchanged between counsel for the co-defendants and concerned a witness interview. *** The Court finds that Plaintiff has met its burden to demonstrate that the December 6, 2005 emails are protected by the joint defense exception to the waiver rule of Colorado's attorney-client privilege doctrine.