Asking questions is the converse of contributing comments; essentially, asking questions is a way of encouraging the members to formulate their own explanations. By asking for clarification whenever one fails to understand something, the student will gain useful insights that he might not yet have considered.
By arranging his confusion into a question, any inconsistencies in the student's thinking can be pointed out to him, and the assumptions implied by his question are at the same time exposed to the scrutiny of the group.
Furthermore, because of the diverse personalities in study groups, each individual will likely have something unique and useful to contribute to the group; and by asking questions, these unique traits can be encouraged to surface.
A good way of reviewing one's knowledge is by explaining the concepts involved in the subject matter, especially to a study group, thus it is advisable that students take the responsibility of teaching each other.