Sometimes a student will sit back and let others do the work and thinking. Sometimes I see students that do not want to help anybody. They look after number one. I do feel that many individual members of the group do not take the responsibility of individual learning thus reducing their skill and understanding of mathematics. They depend too much on what the group for finding the answers.
Some students use group work as a way to avoid actually doing mathematics.
Some are even able to seem very productive within their group while doing this, by taking charge of things like setting up group meetings, organizing and collating assignments, etc. The negative behaviors have usually been related to grouping and its logistic problems and not towards mathematics itself. In general, negative attitudes are of the 'you're supposed to teach me' variety. With lots of little groups, it is too hard to answer everyone's questions. Unless the class is very small, students spend too much time with their hands in the air, waiting for me to help them, while I'm busy with another group.