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