Halo 2 was basically synonymous with Xbox Live and everything that came with it when it launched (for better or worse). For example popularizing voice chat (including proximity!), rapid matchmaking, persistent parties between matches and gamemodes and player ranks and levels. They talk it about it a bit in the second half of this video.

https://youtu.be/YGSuPZVgxLg?si=cQZRaXJGaGFaKuL-&t=172