The detection, coordination and response capabilities of critical infrastructure operators ultimately determine the economic and societal impact of infrastructure disruptions. Operational cyber security exercises are an important element of preparedness activities. Emulation testbeds are a promising approach for conducting multi-party operational cyber exercises. This paper demonstrates how an Emulab-based testbed can be adapted to meet the requirements of operational exercises and human-in-the-loop testing. Three key aspects are considered: (i) enabling secure and remote access by multiple participants; (ii) supporting voice communications during exercises by simulating a public switched telephone network; and (iii) providing exercise moderators with a feature-rich monitoring interface. An exercise scenario involving a man-in-the-middle attack on the Border Gateway Protocol (BGP) is presented to demonstrate the utility of the emulation testbed.