In tight formation, unless you are at the front of the bunch while cycling, you have limited visibility and it is difficult, or even not possible to see what is happening up front. Of course this creates a risk situation for you as a cyclist and so bunch riding rule and etiquette need to work in your favour to help protect your safety.
Communication in a bunch is through hand signals and calls (shouts). Often calls and signals are initiated by the riders at the front, but not always. Riders at the rear or in the middle can also make calls which are important to the safety of the entire peloton. The hand signals and calls make up for the impaired vision out front and should provide all of the information you need to ride and react safely in the bunch.
Regardless of who makes the call, it is crucial that every single rider passes this information on. Both voice calls and hand are repeated by each rider. In a bunch there is a lot of noise, the sound of bikes on the road, traffic and even the sound of the wind which can make a call by a rider a few lengths ahead or behind inaudible. Because sound is easily lost, it is crucial for each rider (and not just every second, third or fourth) to repeat a call and ensure that they need to slow down or stop or avoid an obstacle.
Some calls such as STOPPING have a hand signal which is supported by a verbal call. In the case where a rider needs both hands on the steering wheel, and it is unsafe to make the hand signal, the audio call fills in the gaps. On the other hand, some information may be passed on by hand signals alone, for example a parked vehicle ahead is signals with a hand behind the back and can provide sufficient information as the bunch slowly moves over to pass.
For more on bunch riding etiquette and safety, download the Bunch Riding guide.