What does this blue symbol mean?

I have just done a variety of events in the Straits of Devastation with 30-40 random people and upon finishing I noticed that a few of these had a blue symbol above their heads, kind of like an arrow pointing down at a chevon:

I am assuming that this is something to do with the auto-grouping feature in Guild Wars 2. What does this blue symbol mean? Is this something that is assigned by the game or player assigned?