We are a company of 6 and use Microsoft 365 Business Premium. Active Directory is technically included in all the Office plans as far as I know as that's just what's managing user accounts, but the Microsoft 365 plans include it with InTune which is more in depth, such as allowing logging into Windows 10 devices with it.

The Microsoft 365 licences are per user, not machine. You would generally get a Microsoft 365 licence for each employee so that they have their own email address, can use Teams for instant messaging, share files on Teams/SharePoint and so on. You can mix and match licences so it's less than $10 per person so very inexpensive.

It seems like there's 2 things here, 1) you have a few users and want to share documents, files centrally, while being able to lock out access or keep access if someone quits, and 2) that you generally share a couple computers for multiple users.

So first you'll need a Microsoft 365 plan for each employee. That way everyone has their own email address and login, can use Teams to instant message, share files, use SharePoint and so on. You can control access, such as restricting some employees such as managers to management only folders on SharePoint as well as being able to lock out a user if they quit. You can mix and match licences as well. There's Business Basic, Business Standard and Business Premium. Business Basic gives everyone access to Word, Excel, PowerPoint and Outlook on the web only, which is good for shared computers. Business Standard includes the desktop versions of Word, Excel, PowerPoint, Outlook. If your users are all sharing a couple computers then Business Basic is probably better as they'd be using their email on the web anyway vs in Outlook desktop.

The above works if everyone logs into a Windows 10 or other computer with a local account. If you wanted the users to be able to log into the desktop computers in the office with their Microsoft 365 credentials, then you'd need Business Premium which includes InTune which allows you to manage devices as well.

I'd suggest doing a trial of Business Standard with a licence for each employee and get started with that first, then potentially look into InTune. But based on your details provided I'd also recommend reaching out to a Microsoft partner if you wanted to get more in depth assistance as it's a great product but hard to fully explain in a simple post. :)