Something like:
Public Declare Function WNetGetUser Lib "mpr" Alias "WNetGetUserA" _
(ByVal x As String, ByVal UserName As String, IBufSz As Long) As Long
Dim BufSz As Long, result As Long, Buffer As String
Dim NullPos As Integer
BufSz = 256
Buffer = String$(BufSz, 0)
result = WNetGetUser("", Buffer, BufSz)
If result = 0 Then
NullPos = InStr(Buffer, Chr(0))
GmsGetUser = Left(Buffer, NullPos - 1)
End If