It is often claimed (here, too) that "anyone may sue." This is true. Just pay a filing fee, etc., and anyone can sue. Alice can sue Bob because he is reading "War and Peace." I can sue Suzie because I didn't like the color of the dress she wore. And so on, so the theory goes.
YMMV, depending on locality. Some places require a lawyer to certify that the lawsuit has (in hir opinion) at least a reasonable chance of succeeding. That may not apply in small claims court, tho.
(And the courts have even taken steps against those who have filed many frivolous lawsuits with the apparent intent of harassment. )
They can go after the aforementioned lawyers for this, too. One of Scientology's lawyers, Helena ("The Kobra") Kobrin, has been sanctioned for filing frivolous lawsuits on behalf of that cult.