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).
People often behave in the same manner, doing one thing in unison with the other members of their group. When these people are part of a collective noun, that noun becomes singular and requires singular verbs and pronouns. Every afternoon the baseball team follows its coach out to the hot field for practice. Team = singular; follows = a singular verb; its = a singular pronoun.