They are just there to server as dividers so you can easily see how many people are on your team etc. It is 4 non-highlighted, 4 highlighted, and 4 non-highlighted in a 12 person team.
Does the order from top to bottom mean anything? As in rank or anything?
I seem to be placed higher on the list immediately following a match which I did really well on. Could be coincidence too, just wondering if anyone knows.