If you already know who you are looking for, you can find them directly through the console, mobile app, or PC. : Press the Xbox button to open the guide. Navigate to People and select Find someone .
If you want to check a user's stats without adding them, sites like XboxGamertag.com or TrueAchievements allow you to search for a Gamertag publicly. These sites will show you: xbox user search
If you don't have a specific name in mind, use these community tools to find active players: How to add new Xbox friends If you already know who you are looking