Rule 1: "Use 'T' Button.
With T you can mark Positions on the Map for your Teammates. This will look like this:
Markers
Everyone in your Squad can see your Markers. And everyone in your Squad can place these. As you can see you have 3 colors. Use them wisely!
(Also I highly suggest you rebind this button to TAB)
Build Help
Tell them where to regroup. Where to go. Where to make line. Tell them each and every step. It will have a big improvement to your gameplay!
Instead of saying the name of the person to assisst, how I do it, I place Red marker on him and shout "Shoot to Red Marker". You know why this is 100 times better? Because Bursters standing behind smoke don't have to see enemy, they just burst into red markers. VERY VERY STRONG!
Scout (Which is Sneaker) will use these Icons as well to tell you where enemies are, etc. etc. Use information wisely and try to make good decision.
You have to use as much Gameplay mechanics of this game as possible to succeed. Enemy has a lot of Snipers you can't beat? Wait for night-time where Perception is reduced by huge amount and start capping zones during night! The more late it is, the more perception malus there is. Biggest malus to perception is at 1 O'clock ingame time.
Have enemies that manage to kill you at strange angles? Use smokes and cover your movements!