I wondered about this, too.  Apparently all these women had worked for Sholto, but none currently did, or Sherlock would have recognized the name.  He asks to see if they all work for the same person, and it turns out they don't (now) and probably didn't at the same time.  He says something at the reception, when he's figuring it out, about frequent turnover in staff for the sake of security (although having known, loyal staff would make Sholto far more safe than bringing in strangers regularly).  But it still doesn't work.  The Mayfly Man met each of these women outside work, and took each of them to "his place."  If he'd gone to their place, he might have seen the wedding invitation on a table or something, but why would they be discussing their employer's comings and goings with a first date, at his place?  There would be no way for him to get the information he wanted without asking questions that would be very strange, in context.