To learn more about Yale alumni, have a look at the Association of Yale Alumni (AYA) and the Yale Career Network. As a current Yale student, you can log into the Yale Career Network with your NetID through the "students"-link.
The Graduate School Alumni Association is the branch of the AYA that is specifically concerned with alumni from the Yale Graduate School.
A brand-new resource at Yale is STAY (Students and Alumni of Yale), which brings together Yale undergraduates, graduate and professional school students, and alumni through service, mentorship, and social activities.
You may also want to have look at the resources of the Graduate Career Services office.