I can tell you based upon the following publication:
"Uniform Painting Guide to the French Indian Wars 1754-1763"
Forces for the Siege of Fort William Henry 30th July – 9th August 1757
French:
Regt Royal de Roussillon (approx 500 men)
Regt de La Reine (approx 500)
Regt de La Sarre (approx 500)
Regt de Bearn (approx 500)
Regt de Languedoc (approx 500)
Regt de Guyenne (approx 500)
Compagnies Franches (approx 1000)
Canadian Militia (approx 2000)
Coureurs de Bois (approx 100)
Indians (1600)
Artillery 30 cannon and 15 mortars
British:
Regt 35th Foot (750)
Connecticut Provincial Regt (400) 1 Btn
Massachuetts Bay Provincial Regt (800) 2 Btns
Artillery 17 cannon 7 mortars
Civilians 100+
I hope this helps!
Forgot to add for the forces who amde an attack in March 1757 of Fort William Henry
French: (1600)
Compagnies Franches, Canadian Militia, and 2 Indian warbands
British: (400)
Possibly the 35th Foot I will need to confirm this though
but more than likely it was the 35th Foot
.