If your Organization has Global Accounts enabled, any user that has access to the Organization in question will be able to see tasks that are associated with the Contacts and Accounts.

As an example - imagine a chain contains Hotel A and Hotel B. This chain has Global Accounts enabled. A user that has access to both hotels can add a task to their contact, John Smith. A user that only has access to Hotel B will still be able to see the task that has been added to John Smith's contact card as all contact and account information is shared between all organizations within the chain.