Five cases of collaborative information seeking and retrieval are selected under the context of real information needs, and their behaviors data are collected with the methods of thinking aloud, observation and interviews. Its 17 main conceptual categories are coded, and 5 core conceptual categories are focused with the grounded theory. Based on these categories, comparative studies of 5 cases are made on three levels of subject, context and behavior, and the process of collaborative information seeking and retrieval is presented. On the basis of detailed analysis about relevance judgment in collaborative information seeking and retrieval, five major influencing factors are proposed, and a model of relevance judgment is given. The results show that: there is no significant difference in relevance criteria between collaborative information seeking and retrieval and single user one; the main difference between them is group communication; and context takes a great role in reaching group consensus.