We understand that at times you want to to be in contact with a certain user: He or she may be a successful applicant of the previous year, an applicant to the same school or an active user with great sense and experience. They will provide you with useful advice and recommendation.
You can contact them via two following channels:
Comment on their profile page: If your questions are general enough, you can ask it publicly by leaving your comments on their profile page.
Private message: if you do not want your message to be known by the public since the leak of information might hurt your chances, you contact contact privately with a certain user using private messages.