Santa Cruz County says anyone can sign a ballot argument (from
http://www.votescount.com/books/argument.pdf )

“Signers of arguments for or against a county, school, or special district measure do not have to meet the criteria listed above. The filer of the argument must meet the criteria above; however, anyone may sign the argument.”

Shasta county agrees:
“Arguments and Rebuttals must be accompanied by the Verification Statement included in this Guide. (Elections Code §9600) There is a distinction between a “filer” and a “signer or author.” The filer argument or rebuttal must be either the governing board of the district, a bona fide association of citizens or an individual voter who is eligible to vote on the measure. The “signers or authors” of the argument or rebuttal can be any person or organization accompanied by a signature of a principal officer. Filers do not have to be signers.”

Sutter county has almost identical language to Santa Cruz. Butte County also.

San Luis Obispo has different “filers do not have to be signers” language.

Orange County allows signers who are not filers, but requires that the filers formally delegate their right to sign. See http://ocvote.com/election/rebuttal_Handbook.pdf

Sacramento County follows a similar system to Orange County.

Where did this all come from? San Mateo County ROV says that our arguments against local tax measures authored and filed by LSPM’s secretary can only be signed by people who are allowed to author or file the argument. First, the law doesn’t say that. Second, our actual disagreement is about whether we have to use our titles. That is, I’d like to sign ballot arguments as “Kevin Dempsey Peterson, Software Engineer”, because that’s the designation I’ll be using if I run for office again. The ROV wants to put me down as Treasurer, Libertarian Party of San Mateo County. Seems somewhat reasonable, and I’ve have no problem if the law were actually written that way.

Bonus points: the form they provide for submitting ballot arguments misquotes California election code, combining “authors” and “signers” into the same thing (while the EC makes a distinction).