Trevor's advice is correct, with two corrections.
1. Project Professional does not come with a Project Server User CAL. It comes with a Device CAL. This means that Project users may not access Project Server from any device other than their primary device unless they separately purchase additional Device
CALs or a User CAL.
From the January, 2014 Product List:
"Customers who license Project Professional 2013 will be deemed to have one Project Server 2013 Device CAL."
2. The need for Project Server users to also have a SharePoint Enterprise CAL is, as far as I can tell, undocumented in official documents such as the Product List or Product Use Rights. While white papers (and forums like this one) may state it as
a requirement, it is not.
Product Use Rights identifies the following functions in SharePoint that require a SharePoint Enterprise CAL. The list does not include Project Server functionality.
"Additional Functionality:
Business Connectivity Services Line of Business Webparts
Office 2013 Business Connectivity Services Client Integration
Access Services
Enterprise Search
E-discovery and Compliance
InfoPath Forms Services
Excel Services, PowerPivot, PowerView
Visio Services
PerformancePoint Services
Custom Analytics Reports
Advanced Charting"