You can share the activities in which the multiple resources are involved.
While creating the activities in which multiple resources are selected, the activity will automatically be assigned to the resources and those activities will be displayed in their calendar as well.
In appointment activity, one needs to mention required and optional users while in case of other activities it appears as from and to.
Note: For activity to be visible in multiple calendars, users should be selected in the look up record option.
All the activities can be shared with multiple resources except ‘Task’ Activity.