First player He plays a vital role for your team, but it can be quite challenging. He will listen for footsteps towards underpass or in B apartment. Other than that he'll make sure no one can enter through connector without your team knowing. The challenging part is to know when to let go of connector and assist your mate over at B. Just don't forget to let your team know that connector is open when you do.
Second playerLike I mentioned earlier, I don't like to peek from window room unless it's a quick peek and you spawned close. Aside from the smokes and molotovs, you risk getting killed early on. If that happens the terrorists can push whichever site they want, knowing that the defenders from the other site will be late to the party. Instead I like to have the mid guy in window room peeking catwalk. If he gets smoked off he can wait for a few seconds and then put down a molotov on cat and then decide if it's best to rotate towards connector, market or to simply stay put. Should the terrorists on the other hand choose to go aggressive towards connector, he can use this pop flash to peek and hopefully secure a kill.
Third player Generally I like to play one player on the B-site, whose job it is to spot enemies and call for backup. I would advise against peeking from the van unless you have an AWP and are ready to fall back after the initial shot. It's still a risk due to how easy it is for terrorists to rush down a solo AWPer on B. I would suggest that you try to go for jump peeks from time to time. Even if they're ready for it you're difficult to hit and you'll get loads of information!
last two playersThe last two players are obviously the ones on A. It's rare to see an A-push without smokes towards CT spawn, jungle and stairs these days. That makes it a walk in the park for terrorists to isolate defenders on A-site, which is why I'd recommend peeks from far back. CT stairs, danger box and the area around stairs are excellent spots. Once you see the smokes come in you should call for backup and then assess the situation. If they don't seem to push you can tell your teammates to go back to their previous positions. If they however do push you can try to use the smokes to your advantage. Call for a flashbang onto site from either connector or jungle and then jump up on ticket.