Just enabling Copilot globally isn’t enough.
When you join a Live Share session, VS Code runs a second extension host for the shared workspace, where extensions start disabled.
• Open Extensions ➜ search GitHub Copilot ➜ click the ⚙️ gear and choose Enable (Workspace) (or Install in Workspace).
• Reload when prompted and sign in.

Each guest needs their own Copilot subscription; once the extension is enabled in the workspace, both inline completions and Copilot Chat work normally.