There are a few ways to communicate between players within the game. In addition to the in-game communications (calls and text chat), players have adapted certain controls in order to allow communication without taking the vital control away from the dinosaur when typing in chat. They are not official, but have become common signals in the game that other players follow.
The game inherently allows for a selection of distinct sounds to allow players to communicate; these are mapped to the number keys (1, 2, 3, 4).
A general sound to signal to others "Here I am!". Depending on which species answers the call, this can be good or bad. Try to use this sparingly.
Communicates that you are not going to kill a player. Generally used by carnivores to signal that they aren't looking to fight, and used by herbivores to greet other herbivores.
Communicates that an angry or aggressive reaction is likely. Used in confrontations between players.
Communicates that you are in need of help, or have spotted a predator. Usually used with social animals.
The 'Peace Dance'
Rapidly pressing the crouch button is a common method of indicating non-hostility among all dinosaurs that can crouch, making it a universal signal that no confrontation is imminent. The dance is often accompanied by the dinosaur's friendly call.
Commonly seen in Tyrannosaurus players. When two T.rex spot one another, they will generally pace around, examining each other and determining if the other is a threat. Threaten roars are traded during this; this behavior ends in either a direct battle between both tyrannosaurs or in one of the pair moving on.
'Yes and No'
Not many use this method, but it can also be used to communicate, as all dinosaurs have access to it. By manipulating the camera, players will make their dinosaur nod or shake their head in reaction to certain events.