Now you are saying again that any user of any computer on the same network can access your computer even if not in the same workgroup? How?
Because the Workgroup system has no control over sharing. It is just there to group computers to make larger networks easier to navigate, not to control sharing of resources. Windows network sharing permissions are Username and Password based, the Workgroup has nothing to do with permissions.
I'll give you an example using two computers.
Computer1
Computer Name: Computer1
WorkGroup: Workgroup1
IP Address: 192.168.1.100
Username: User1
Password: Password1
Computer2
Computer Name: Computer2
Workgroup: Workgroup2
IP Address: 192.168.1.101
Username: HLJOIJKMOMIOJ
Password: HOJMKOMHOMKIHOIH
Now, we'll say Computer1 is sharing resources, say a printer and a folder containing some files. Computer2 can access all of those shared resources. Type \\192.168.1.100 into Explorer, it will ask you for the username and password of Computer1, after you enter those you get access to the shared resources. The Workgroup has nothing to do with that. You can also type \\Computer1 and get the same thing, again the Workgroup doesn't matter here. This is seriously Windows networking 101. Even if Computer1 has network discovery turned off, it will still show up this way.
Sure, you can say "but you have to know the other computer's IP address or computer name". Yep, and there are an insane number of free network scanners that will give you that information.