Data communication between a group of users

When an ad hoc group of people come together electronically to perform a collective task they have shared responsibility and need to be accountable. The invention provides irrefutable binding of group data to group members. A number of users with associated computing devices form an ad hoc communication group via a messaging server. The messaging server stores data associated with the ad hoc communication group including digital commitment signatures from users in the group and a public group signing key. The digital commitment signatures irrevocably and verifiably bind the respective user's membership to the group. The public group signing key is used to verify group data outputs signed by computing devices in the group. Each digital commitment signature may be generated based on a user and group specific commitment text that is signed by the user's private signing key. A quorum of group members may be required, that is a predefined minimum number of members have to cooperate together to produce the digital group signature.