Windows Server 2012 RDS - How to add new Domainuser

So far I never had much to do with server administration, so this question might be very basic to some people.

I have to manage a Windows Server 2012 R2. There are around 8 users who use it. They all have a local computer and connect to it via RDP and work on the server. If i type in cmd

net users

there are only the standard users (Administrator, Guest and 1 more costum), but none of the 8 "real" users. If i create a new user with

net user newuser newpw /add

this user appears in the "net users"-command, but has a different domainname than the 8 real users. For example newusers fullname is "ITS/newuser", while all the "real" users are "COMPANY/username". The Server is not member of an Active Directory Network.

Furthermore when I log in as Admin and use the Computermanagement-Tool, i canot find the 8 real users listed anywhere. The 8 real users all have a folder in C:\Users\username with the typical subfolders like Desktop, Downloads, Documents, etc.

I want to add a new user, and put them into the same domain as the 8 existing users. So far, I have no idea how to do this.

Of course i googled a lot, but every solution I found was about managing users in an Active Directory Network and in my case there is no Active Directory.

Can someone explain in general what exactly is going on and how this server works? To be honest I do not even understand the structure about how users are managed on this server and I am desperate for any information.

If you want more information or even screenshots, just ask for them. I am very glad if someone can help me

net user user /ADD is the correct way to add an account, the format should be; MACHINE\username. If you see COMPANY\username then the net user was run against your Active Directory.

Please use computer management to manage the local user database, it's way more simple than using net user. Run->compmgmt.msc

If you can't see the user in compmgmt.msc, it mean you runned net users without the /ADD, which doest add the user, or net user was run against the DOMAIN.

For that I suggest to use computer management it's less confusing.