Another quick google-search-turned-post to help others looking for a solution to this problem.
After my last Steam upgrade (Mac client) the community overlay stopped working in Team Fortress 2. This breaks a number of features, including the "Invite" button when creating a party. This is the solution that worked for me:
1. In Steam Library, right-click Team Fortress 2 and choose Properties
2. UN-CHECK "Enable Steam Community In-Game"
3. Quit Steam
4. Re-open Steam
5. RE-CHECK "Enable Steam Community In-Game"
Overlay starts working again!