You can use Application.Visible = False to hide Excel. Here is an example, in
the UserForm_Activate event of the form:

Private Sub UserForm_Activate()
'Hide Excel when this form  is activated
Application.Visible = False
End Sub

Use Application.Visible = True to make Excel visible again. Here is an example,
in the code attached to a "Close form" button on a userform:

Private Sub cmdExit_Click()
Unload Me
Application.Visible = True
End Sub

Or, use: Windows(1).WindowState = xlMinimized
in either ThisWorkbook in the automatically-open part of the code,
or the userform code.

You may need to change the "Windows(1)" part to fit your situation,
since the window  a user may be using may not be the first one
running on Excel.

And, remember to open it again with something like: Windows
(1).WindowState = xlMaximized if you need the spreadsheet  needs to
be viewed later.