Timekeeper: Someone needs to make sure that the group stays on track and gets through a reasonable amount of material in the given time period.
Summarizer: Every so often (perhaps once per question for a list of questions, or at the end for one question), this student provides a summary of the discussion for other students to approve or amend.
Leader/Editor: This student is in charge of organizing the final product of the project, be it a paper, a presentation, etc. That doesn't mean technical details, but of making sure that the project meets the standards set out by the instructor (often as a rubric), plus any extras stipulated by the group. These standards generally include punctuality and completeness.Recorder/Secretary: This person takes notes whenever the group meets and keeps track of group data/sources/etc. This person distributes these notes to the rest of the group highlighting sections relevant for their parts of the project.
Checker: Someone needs to double-check data, bibliographic sources, or graphics for accuracy and correctness.