OK, every school is a bit different and my school has changed things a bit over the past decade but this is roughly how it goes:
There are about 30-40 reviewers. Most are faculty members, some are upperclassmen in the med school. Each application is reviewed three times before interview. The initial reviewer reviews and makes comments, the second reviewer reviews the first reviewers comments and the application and makes more comments, then the file goes to someone with the power to make an interview offer (that is a paid position of considerable power but recommending an interview after 2 reviewers say "no" takes balls and isn't done very often -- otherwise, questions are raised about why these counterintuitive decisions are made). If the first reviewer is very good at identifying a select group for interview and making a good case for them, that reviewer will be very influential in the process as others are likely to go along with the assessment. On the flip side, when such a reviewer recommends "no interview", the application is half way sunk, despite 2 additional reviews.
After interview, the reviewers work in teams of 10-15.
After interview, the file is reviewed by every member of the team and each comments independently about the file and classifies the application (admit for sure, maybe admit, waitlist, ick) The team then meets and discusses discrepancies (why the "ick" when several though this was an "admit now"?) and each indpendently assigns a rating. These ratings are combined and provided to a smaller group of senior faculty (usually 5-9 people) that receive reports from all the teams and make the final decisions. (The members of the smaller group also serves as a team leaders when the teams meet). A majority need to approve an offer of admission or other disposition of an applicant so no one person has the power to admit or deny. Sometimes, a single vote makes the difference but most are unanimous decisions.
Membership on the adcom is by invitation and it is actually, officially, an appointment by the Dean. In truth, the admissions office begs for volunteers and then sends the names of the volunteers to the Dean for appointment. It is considered "good citizenship" in the university to serve on a committee and so some of us do it. Plus it can be fun to learn about applicants and useful to faculty members with kids who want to know more about admissoins (my kids have no interest at this point but what I know has helped understand college admissions). Much of it is drudgery and it takes hours each week to get through the reviews (mostly at home) as well as the frequent group meetings. We don't get paid. We might get some modest refreshments at meetings or a modest gift at the end of the year. I have no idea how they manage to balance the group but it always is balanced. Some of us serve for years and others are appointed for 3 yrs and not reappointed (sometimes because they don't seem to "get it" or aren't willing to carry their share of the load).
Med students read applications and recommend interviews, review applications after interview and make admission recommendations, and discuss the recommendations and then pass those recommendations onto the small group. They work together with faculty at every step.
Other schools are different and have the interviewers present the applicant's file to the group that votes up or down (or waitlist), or have some other mechanism for deciding how to make interview invites. The rules for med school accreditation do require that admission decisions be made by a faculty committee and there were some fireworks a few years ago when a Dean went over the committee's head at one school and admitted an applicant with political connections. (In the end, the Dean's head rolled like the sacrifical lamb that he was but the guy with the connections matriculated.)
