Whats the function of a glycoprotein in phospholipid bilayer?

glycoproteins are simply proteins with sugar component that are part of the phospholipid bilayer.
Their big role is "body recognition" They are part of our immune system and help our body determine what is ours and what is foreign. It is like the "stamp of approval" our body gives our cells.
An example you can think of is our blood system A, B, AB and 0.
This system is coded in genes and expressed via various glycoproteins that are part of our red blood cell phospholipid membrane. If our body has A blood group (determined by A glycoprotein in red blood cell membrane) and encounters red blood cell with B glycoprotein, our body will destroy this B glycoprotein red cell - just like any foreign substance.

Another example of glycoproteins are the antibodies that out body produces to fight foreign bodies such as bacteria and viruses.