We need to setup partner community so that multiple users belongs to same account can see each other’s Tasks. In other way, if partner user has access to parent related record then he must be able to access the related Tasks whether he is owner of Task or not.
By default, user who owns the Task only can access it.
There are two ways to achieve this. Below given both workaround assume that Partner User has Edit Task & Activity Object Access at profile level.
A global Configuration using Sharing Setting:-
a) Go to Setup –> Sharing Setting
b) Edit Organization-Wide Defaults and go to Activity object.
c) Make Default External Access as Controlled by Parent.
Standard “Public” Field on Task to share the task at record level
There is standard Public field on Task, if Public field in Task object set to “true” then external user can access this Task specifically.