The search bar on the Friends List allows players to search for their Friends by their Niantic ID, display name, or in-game username. All in-game username searches respect the Visibility Settings you set for the game. Visit our article about Game Profiles and Visibility Settings for more information.
When a Friend cannot find you by your in-game username, it's usually because your game profile Visibility Settings do not allow your in-game username to be shown to other players when searched. Go to 'Edit Profile' in your settings or directly from your profile and update your games' visibility by selecting 'Friends' or 'Everyone' from the dropdown menu to allow these groups to search for you by your in-game username.