In the list of plans for every group, there is a plan titled "No Plan Activities." This is a pseudo-plan of sorts, the configuration of which controls the sync'ing behavior of Activities that do not have a Plan Association. 

This plan is automatically created as a Default Plan when a new group is created. Thus, every non-coach/admin user typically has a subscription to this plan. This insures that every non-coach/admin member added to the group will have Activities created with no Plan Association sync'ed to their calendars. You can, of course, change this behavior by revoking its Default Plan status and/or removing individual group member's plan subscriptions to this plan.