You'll want to use xpath constraints in the entity's access rules, so that users can only see tasks that were assigned to them.
For how to add xpath constraints to entity access, see this page in the documentation:
At the view page for the tasks for the users make the data source Xpath and please follow the bellow one :
[the association between the task and the account/the account entity/id = '[%CurrentUser%]']
then each user can see just the tasks that assign to him.