Send notification to newly added team member at the time of they added.
it is observe that information related to given task was not shown to all team member except assign member, and also this plan was not shown to all team member in planner hub.
This plan shows only when the task is assigned to particular member.
So, i suggest you to send notification for newly added team member. when this notification given to the member then plan will show in planner hub. after that any member can see this information through planner hub.