Some students are motivated to study in a study group

because the other members likely …

encourage them to study
On the positive side a study group gives the students motivation to study and update their readings because the other members likely give them encouragement. It also helps them to learn new ideas as well as important points they missed on their readings. Most of the members in my study group are very open in sharing what they know. These alert me that I should update myreadings so I could also share something.
  ... that the bill be endorsed.