Ultimate way to communicate while riding, an offline walkie talkie app?
Have you tried talking with the people you ride with? If traffic is too noisy to understand each other, look for different routes. What is more, such busy traffic is not the time and place for chit-chat.

On a moderately quiet road I find it easy, and pleasant, to chat while riding side by side.

When riding in a file yelling and gestures work well. If you go on a group ride it might be a good idea to agree on some useful phrases. Select them in such a way that similar phrases sound distinctive.

There are a few hand signs you may want to learn. Typically one uses the hand towards the centre of the road to indicate. Most symbols are displayed low, just beside one's buttocks.

Stop:
Flat hand up with bent elbow (not in USA, might indicate turn)
Hand down, palm facing rear.

Pot hole, obstacle:
Finger points, down, at line that leads into the hole.

Slow:
Flat hand with palm facing down, wrist extended, elbow angled, hand and lower arm parallel to ground, move hand slowly up and down.

Here's a video explaining some symbols: