Supported crossplay platforms:

Forza Horizon 3 is supporting cross-play across multiple platforms. Currently it is possible to play together with your friends on Windows PC and XBox One.

Summary:

You’re in charge of the Horizon Festival. Customize everything, hire and fire your friends, and explore Australia in over 350 of the world’s greatest cars. Make your Horizon the ultimate celebration of cars, music, and freedom of the open road. How you get there is up to you.