Halo 5: Guardians doesn't support splitscreen gameplay with your friends, but you can still play online with them via Xbox Live. Follow these simple steps in order to do it:
In case you don't complete the team of four, AI bots will fill it, using characters based on the team you will play in (Osiris or Blue). Each of them has a specific role, like sniper, support, tank. The protagonist, Master Chief or Spartan Locke, is usually the most basic character in terms of gameplay.
Co-op mode will be as valid as single-player missions when it comes to progression, meaning you will only have to play once to unlock achievements and progress through the story.