Is there a term for people spelling out a name or some other word with their bodies (e.g. YMCA), usually for the purposes of a fun photo? This is similar to sign language but uses the whole body with arms and legs.
As far as I remember, it's called Acrobatic Alphabet.
N.B.: After some googling, I've found the term Human Alphabet. You can check out the results from this link.
Santas in Star Formation: Schoolchildren dressed as Santa Claus form the shape of a star ahead of Christmas celebrations ... [Source: Reuters Date:12/28/2011]
In various contexts not limited to using the human body alone, this pattern is commonly known as a formation. We usually see aircraft flying in formation.
Formation flying is the disciplined flight of two or more aircraft ... In civil aviation, formation flying is performed at air shows and is also flown for recreation.
Formation skydiving is the art of building formations in free-fall with multiple people gripping each others' limbs or specially built "grippers" on their jumpsuits.
A V formation (sometimes called a skein) is the symmetric V-shaped flight formation of flights of geese, ducks, and other migratory birds. V formations are also used on military flight missions.
Crop circles are also referred to as crop formations, because they are not always circular in shape.
In association football, the formation describes how the players in a team are positioned on the pitch.
Formation dance is a style of ballroom dancing. It is pattern or shadow team dancing by couples in a formation team.
[EDIT-1: As suggested by @Mehper C. Palavuzlar] Acrobatic Alphabet / Human Alphabet may be more appropriate depending on the OP's question.
Alphabet Recreation has all the alphabet formations in photographs.