The SUMSQ function syntax has the following arguments: Number1, number2, ... Number1 is required, subsequent numbers are optional. 1 to 255 arguments for which you want the sum of the squares. You can also use a single array or a reference to an array instead of arguments separated by commas.
THEY/THEIR (SINGULAR). Using the plural pronoun to refer to a single person of unspecified gender is an old and honorable pattern in English, not a newfangled bit of degeneracy or a politically correct plot to avoid sexism (though it often serves the latter purpose).
A collective noun is singular. Jack is correct. The grammatical explanation is that words like everybody everyone are considered to be collective nouns and treated as a singular entity, a group, a singular noun. The use of a singular noun should always have a singular verb and any related possessive adjectives should also be singular (like his or her).