Why older pages are confusing
Multiplayer appeared in earlier marketing, trailers and user tags. The September roadmap places its introduction in a later update. Store tags describe interests and plans; they do not override the milestone announcement.
What is planned with it
The Backstabby Update describes competitive or cooperative play, rivalries, alliances, challenges and victory conditions. It also pairs multiplayer with an economy expansion and a community-voted city. No date is assigned to this milestone in the inspected announcement.
Planning a group campaign
Wait for released patch notes before organizing a multiplayer session. Connection steps, lobby settings, save compatibility and crossplay rules need a shipped implementation. A grayed-out demo control is not an invitation method.