A method and apparatus are disclosed for controlling collaborative access to a work group document by the users of a computer system. A combination of public-key cryptographic methods, symmetric cryptographic methods, and message digest generation methods are used. The document has a data portion and a prefix portion. A computer-implemented collaborative encryption method uses structures in the prefix portion to restrict access to the information stored in the data portion. Users who are currently members of a collaborative group can readily access the information, while users who are not currently members of the group cannot. Other structures in the prefix portion support collaborative signatures, such that members of the group can digitally sign a particular version of the data portion. These collaborative signatures can then be used to identify the signing member and to determine if changes in the data portion were made after the collaborative signature was linked to the document.