Unfortunately, cross-platform play between Xbox and PC versions of the game is not possible at this time. We did want to include this feature, but development limitations prevented us from doing so.
An article was published on the internet by a website called xboxachievements that stated cross-platform play between Xbox and PC was possible - this was an error. We contacted the website in question and asked them to change their article, which they have now done but Google still has an excerpt of the cache'd article which has caused some confusion amongst players.