So you have a few windows desktops and you need to have a centralised user account server. You don't have to but licenses for Windows Servers you can host Active Directory for free in Linux. Sure it is your choice but why splash the cash when you don't need to. In this video we install and configure the Samba 4 Active Domain Controller on Ubuntu 18.04 and join a windows desktop to the domain.
Additionally you can find my video courses on Pluralsight: http://pluralsight.com/training/Authors/Details/andrew-mallett and take time to see my own site http://www.theurbanpenguin.com