A Study Group Leader (SGL) differs from a TA or PLTL Workshop Leader in that they are not presenting any course material or grading any course assignments. Instead, the student leader focus on building connections between the students who attend their sessions each week and encouraging them to study together outside of Study Group sessions, too.
SGLs are employed by the Learning Center and are not part of the instructional staff of a course. In particular, because Study Group sessions are voluntary and ungraded (i.e., not part of the course they support), the students who attend can be fairly certain that the other attendees are committed to consistently studying for the course, which helps bring together students with similar academic goals who might not otherwise have met in the larger lecture sessions or even their recitations, labs, or PLTL Workshops.
SGLs are specifically trained NOT to answer homework questions, and they instead help students with things like synthesizing their course notes, quizzing one another, and other collaborative studying techniques. As such, having a SGL does not replace the regular office hours offered as a part of your course or any review sessions you might organize before your exams.