Well, I generally change the name of all of the controls on my forms, so that the control name is not identical to the field that is bound to it. So I would recommend changing the name of the textbox to something like:
txtArgumentGroup
and refer to it as:
me.txtArgumentGroup.SetFocus
But if you don't want to do that, then you must wrap the name in brackets.
me![Argument Group]
But when you are referring to a control, you should use the dot (.) syntax, not the bang (!). I only use bang when referring to a field in a recordset or which exists in a forms recordsource but does not have a corresponding control on the form.