If their paper is under discussion, let them know they don't need to sit passively, but can direct the flow of talk. They can ask for help with certain problems, request follow-up information, or draw out silent members. This is their time to use constructively. It helps if each group member feels responsible to bring at least two ideas for the person whose paper is under discussion. Insist they be both respectful and frank.
Few groups are too brutal.
Instead students can bog down in being so nice and polite that they are no help to one another. Look back at the criteria you set out for your writing assignment and have students look for the same issues in the writing of one another. Then, have the group members comment on this issue. Unless it's an editing session, have students focus on substantive writing issues dealing with content and organization rather than on grammar and punctuation. Keeping Students Working in Groups on Task Groups work best when given specific tasks to do.