The goal of the mentor system is to help integrate new students into the department. Each incoming student is paired with a post-prelim student, who mentors the new student for the first two years of grad school. The mentor and mentee meet several times each semester to discuss how classes, teaching, research, life, etc. are going. The grandmentors assign the mentor/mentee pairs, organize group meetings, and send reminders to mentors.
In late April/early May, the grandmentors ask current students to volunteer to mentor incoming students. To try to make successful matches with the incoming students, the grandmentors ask the previous year's prospective hosting committee about the new students' academic interests, personalities, etc.
Mentors contact their mentees via email in July to introduce themselves, explain the mentor system, find out when they will be arriving in Berkeley, and open themselves to whatever barrage of questions the incoming students wish to volley.
Whenever the mentee arrives in Berkeley, their mentor should be around to welcome the new student. (If the mentor is unavailable, a grandmentor or another willing student should do this instead.) The mentor shows the new student around the Department, helps the student with residency, paperwork, and other such hassles, and in general tries to make sure that the mentee settles in as smoothly as possible.
Just before the semester starts, the grandmentors organize an all-class orientation and lunch. At the orientation, the remaining organizational loose ends are dealt with as well as possible. The lunch celebrates the arrival of the new class and marks the beginning of their journey as BADGrads. (Yes, I really just wrote that.)
Over the next two years, mentors and mentees will meet occasionally, as appropriate, and have all-class events even more occasionally. The grandmentors will arrange some meetings, but in general it will be more and more the responsibility of the mentors and mentees to get together as they see fit.
As the mentees pass their prelims, they are considered to be fully “mented”, and are ready to go out and mentor the next generation of students themselves. Thus the cycle of mentoring restarts, ready to continue again and again, propagating wisdom and guidance for all of eternity.