User ID is not tracked by default. You will need to generate your own unique, persistent IDs and consistently assign those IDs to your users. If you don’t, Analytics will fall back on the device ID method which is cookie/app-instance dependent.
Once you have implemented your User-ID tracking, then you switch on the reporting in the Admin settings.
Find more information on the Google support page here.
Note: you will need give your end users proper notice about the data you are collecting and obtain consent or allow them to opt out. The data cannot personally identify an individual (name, email address etc). More information here.