Im not sure which company would accommodate a group of 12 people. You could try contacting some of the guides below to see what they say. Rick's 2014 France guidebook will have more guides to consider:
D-Day Historian Tours are run by Paul Woodadge, a passionate historian and teacher who takes your learning seriously. He offers regularly scheduled tours in his minivan several days a week (€85/day) and also gives private tours. His “Band of Brothers Tour” is excellent (tel. 02 31 22 28 82, www.ddayhistorian.com, [email protected]).
Normandy Sightseeing Tours delivers a French perspective through the voices of its small fleet of licensed guides. They take individual sign-ups and private groups (€45-morning tour, €60-afternoon tour, €90-all-day tour), and will pick you up anywhere you like (for a price). Because there are many guides, the quality of their teaching is less consistent--guides David, Olivier, and Karinne get the best reviews but only do private tours (tel. 02 31 51 70 52, www.normandy-sightseeing-tours.com, [email protected]).
Vanessa Letourneur and her colleague Sabrina are capable, likable French natives who can guide anywhere in Normandy. Vanessa, who worked at the Caen Memorial Museum for years, offers half-day tours from Bayeux or Caen for €60/person (mobile 06 98 95 89 45, www.normandypanorama.com).
The Caen Memorial Museum runs a busy program of half-day tours covering the American and Canadian sectors in combination with a visit to the museum. This option works well for those who have limited time (see museum listing on page *TK). The museum has many guides (some good, some mediocre) and is more likely to have availability when others don’t.