The growing diffusion of wireless-enabled portable devices and the recent advances in Mobile Ad-hoc NETworks (MANETs) open a new scenario where users can benefit from anywhere/anytime impromptu collaboration. However, the development of collaborative services in MANET environments raises new challenges. In particular, unpredictable users/devices mobility, frequent disconnection and reconnection of devices and continuous changes in network topology call for novel middleware solutions to handle properly the communication between transiently collaborating partners. The paper proposes and describes AGAPE, a context-aware group communication middleware, that permits to select collaborating partners, to schedule messages and to tailor their presentation on the basis of group members context, e.g., depending on member's location, attributes, and device properties. In addition, the paper presents an emergency response application prototype to show and evaluate AGAPE functioning.