Who typically serves on a graduate school admissions committee?
At my (large CS) department, the graduate admissions committee officially consists of:


The director of the graduate program.  This is a ~3-year rotating position among the senior faculty.
About a dozen faculty members, distributed roughly uniformly across topic areas and faculty rank.  The department head is not a member; he's way too busy.
About a dozen graduate student volunteers.


However, our graduate admissions database is open to the entire faculty.  Each faculty member is expected to help review applications, and in particular, to identify applicants that they would be willing to advise, offer an RAship, help recruit, and/or recommend for fellowships.  We ask each applicant to name a few faculty they'd be interested in working with, so I usually start by reading the applications that mention my name.  I haven't been on the admissions committee since 2000, but I still read a few dozen applications every year.

For each round of admissions, each research area (theory, graphics, architecture, etc.) provides a ranked list of applicants for their area to the committee, along with estimates of advising/funding capacity.  This usually requires discussion among the area faculty, coordinated by the area chair, who may or may not be a committee member.  (I was the theory area chair for several years.)

Final admissions decisions are made by the official committee, but positive reviews from extra-curricular faculty carry a lot of weight.  In particular, nobody is admitted without at least three positive faculty reviews, including at least one potential advisor.  Decisions are folded back into the database so that faculty can track their favorite applicants' progress, and if necessary, drum up more faculty reviews.

So, in practice, everyone is on the committee, including the department head.