What is the function of cholesterol molecules in the plasma membrane of human erythrocytes?

All cell membrane. Cholesterol is an integral part of the cell membranes and other organelle in all cells. Cholesterol is a structural element in the cell membranes.
Many key functions. Many mistakenly think cholesterol is just something bad that causes atherosclerosis, but cholesterol is essential for our health. It makes up 20-50% of our cell membranes, where it maintains the structural integrity & fluidity of cell membranes & plays a key role in facilitating cell signaling (communication between cells). See http://www.Cholesterol-and-health.Com/cholesterol-cell-membrane.Html.