Can Wii Players Join Xbox 360 Lobbies in Call of Duty: Modern Warfare?

Impossible. The Wii version (Modern Warfare: Reflex Edition) and the Xbox 360 version run on completely separate architectures, servers, and netcode.

• Different builds: Reflex is a Treyarch port on the id Tech 3 engine; Xbox 360 runs on IW 4.0.
• Separate servers: No shared matchmaking infrastructure exists.
• Player count disparity: Wii supports fewer players per match.

Cross-platform play was not implemented in any Call of Duty title until Modern Warfare (2019).