Salesforce: Add to Campaign

Background: A user can add a Lead or Contact to a Campaign via the "Campaign History" related list. 

But the user does not see the "Add to Campaign" button in the list view or report. What is the cause?

Finding: The user needs the following permissions:

• Enable "Marketing User" on User Records
• Campaigns: Read and Edit permissions.

Additionally, the user does not need to have create or edit permission on the Campaign object.