I do not think it is required to have a user license, as the group resides inside a project workspace and the permissions are limited to that workspace alone. However, you can confirm this with SAP. Below is the information I see in the deployment description.
A person is considered a chargeable USER in Ariba Sourcing Professional if that person is in any of the following groups
•Commodity Manager
•Customer Administrator
•Event Administrator
•Junior Sourcing Agent
•Project Administrator
•Project Mass Edit Administrator
•Sourcing Agent
•Sourcing Approver
•Procurement Agent
Regards
Shaaer