Access to resource(s) intended to be shared with specific groups of individuals is controlled using concise tests of shared knowledge instead of (or in addition) to accounts and access control lists. Users can readily learn the concept and choose questions that will control the access by the desired group with little effort. Such questions can be relatively secure to guesses by those not intended to have access, particularly if the number of allowed guesses is relatively limited. Users can generally predict the security of their questions, but sometimes underestimate the ability of attackers to use Web searching or enumeration to discover answers. In such cases, the system can automatically discover weak questions and then suggest alternatives. By lowering the threshold to access control, shared knowledge tests can enable more types of information to acquire collaborative value on the Internet and on other types of networks.