Amicus curiae ("friend of the court") briefs may be filed, with the permission of the court, by any person not a party to the appeal who wants to argue the merits of the case. California Rules of Court, Rule 14. It is advisable to give serious consideration to filing amicus briefs in appellate proceedings.
The Reporters Committee for Freedom of the Press often files amicus briefs in cases involving significant media law issues before a state's highest court.