When you enable an eligible license plan of Office 365, the Microsoft Teams license is enabled by default. The Office 365 individual user license assignment can be seen in the following screenshot:

Microsoft Teams is enabled by default on Office 365 for the whole organization, but if you need to do a proof of concept on a few users or a department, you will need to disable all users that are not part of your proof of concept, or you can use a separate tenant to test it.