You get a bonus for inviting your friends and then they have to invite their friends.
"The game is 4 v 1. The game is heavily reliant on information, which is why Survivors prefer to play with friends in order to relay information to help them in the matches. There are incentives to play in clans with your friends to earn currency to unlock extra rewards."
"Some challenges require you to have friends. Having a few friends is useful as you can earn rare resources by "helping" them They are also useful in completing Dino competitions (Snowman, Fire Mountain etc)."