Sharing Rules cannot be created for Activity (Task and Event) in Salesforce. The Org Wide Default for Activities can be either Private or Controlled By Parent.
1. Private
Only the activity owner (Assigned To), and users above the activity owner in the role hierarchy can edit and delete the activity.
Users with Read access to the record to which the activity is associated (Name and Related To) can view and report on the activity.
2. Controlled by Parent
A user can perform an action (like view, edit, transfer, and delete ) on an activity based on whether he or she can perform that same action on the records associated with the activity.
E.g. If user has only Read access on Account, then he/she can only view the Activities listed under the Account.