So I started IGLing in comp recently and has seen major improvement in my games.
Before IGLing, I had an average of 155 ADR (average damage per round) and a 1.15 KD, but I was losing most of my games (30% winrate) and realized this was probably because of the communication missing while playing.
When I IGL, I usually try to comm these things:

For defense
- Ask a teammate to push or help me with utils (flash, stun, smoke) for early aggression into the round
- Tell where I'm going to position myself so my teammate can play of my contact or ask someone to play in a certain position so I can play of them
- For retakes, I will usually ask for an info utils (can be a dog, prowler, dart, etc) and then flashes or stun to take space

For attack
- If we're fast executing onto a site, I try to give everyone a role and what space they should take, I also call for the use of utils so everyone know what they're doing instead of just blindly running in
- If we're running a default I try and come every bit of info we get about the enemy's positioning and then call a regroup to hit a certain site together
- For postplants, I ask and try to figure out where the enemy players are positioned and try to hold for a cross or swing together

Also
- I like to remind people A LOT when we're up numbers as people usually understand playing for numbers, but don't pay to much attention while playing or forget about it.

Overall, I think IGLing is good for team morale as it keeps people talking and makes the game way easier just in general.