An Azure service that is used to manage and protect cryptographic keys and other secrets used by cloud apps and services.
Azure services are not licenced as such, and certainly not on a per-user basis. You pay for the use of the resource. You will need to create an Azure subscription and then deploy a Key Vault into that subscription. You will be charged for the use of the Key Vault based on the number of requests for secrets you send to the Key Vault. The specific users accessing this do not matter, it is just the total amount of requests.