When you join, at the top of the screen it says the number of players remaining for each team; not saying you cannot open with surprise, but only very inexperienced players/teams will assume only one player accepted the queue.

Hunters do have burst; and as you gear up, yours will improve.
I'm not sure if it is the most burst for any composition or not.
But if you have enough to take out any initial target in a GCD... or in a couple of GCD's if they lack an immunity to damage effect, your opening will give you an advantage.

There are a multitude of compositions in fives.
I've not run 5's yet with my team... but mine is a 4x DPS, 1x Healer composition.
More commonly, I would think you'll find 3x DPS and 2x Healers, at least in 5's.