What's the simplest software to install to allow flexible Windows logins to our Windows 2012 R2 server?

I am the 'accidental admin' for a small but growing charity. We have a Windows Server 2012 R2 running on a single, newish server plus 8 workstations - 6 with Windows 7 Pro and 2 with Windows 10. The single server is simply a file server & not a domain controller. Each user has a simple setup, they are users on the server and have shares that they access as mapped network drives. However with a growth in volunteers I'm being asked to setup the same user on several PCs, so the volunteers have a choice of PCs. What is needed is to upgrade the server software so that users can logon at any PC and access their shares wherever they are. To do this, I guess I need Active Directory installed? My question is: What's the minimum I need to install on the server to make this happen? - and there's a supplementary question of what software needs to be added (if any) to the client PCs? I'm guessing I need to need to make the server a DC and then add AD, but I would value some firmer and more specific information. Are there other ways to achieve this? I want to keep things as simple as possible, of course. Thanks, and sorry if to some people this is a really dumb question.

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