Ideally, send your officiant an invitation as you would to any guest. However, if you choose to verbally invite him or her, be sure to follow up with details and to ask for a response. Never just assume he or she WILL or WON'T attend.
If your officiant does accept the invitation to attend the reception, seat him or her at the head table. If there isn't a head table, seating the officiant with either set of parents is the next best thing.
Often times, your officiant will decline the invitation or simply stay for cocktail hour. If this is the case, take your first post-ceremony picture with your officiant so that you maybe be respectful of his or her time.