When scheduling an activity within an Orbit you can schedule for the Orbit or add contacts as participants. Whether a participant is a member of the Share Orbit or not, the activity will be visible to all or your collaborators. It is not necessary that the collaborators have the contact in their contact list for the activity to be visible.
Completing Orbit-base Activities
To complete an activity from the Orbit activities view, tap on the activity then navigate to the contact record by tapping the contact name. Complete the activity with right swipe. The activity will now appear as “dimmed” in the Orbit activity view.