Go to the bottom menu of the game screen and select Following/Followers to add friends.
In World Flipper, the term "follow" is used instead of "add friends."
[The Following/Followers Menu]
Mutuals - list of mutual followers
Following - list of adventurers who I'm following
Followers - list of adventurers who are following me
Press the Search+ button at the bottom to enter the player ID of the adventurer who you want to follow.
You can use the Share Invitation URL button to generate a link to follow and share it with other adventurers via social media.
Participating in a Boss Battle with a followed adventurer
will grant you various benefits such as exemption from consuming stamina, so try using the follow feature as much as possible!