A person (including a group of persons, business, organization, or local government) commits a federal felony when she or he:and down a bit lower this
* assists an alien s/he should reasonably know is illegally in the U.S. or who lacks employment authorization, by transporting, sheltering, or assisting him or her to obtain employment, or
* encourages that alien to remain in the U.S. by referring him or her to an employer or by acting as employer or agent for an employer in any way, or
* knowingly assists illegal aliens due to personal convictions.
RICO -- Citizen RecourseTo my non-lawyerly eye it looks like a citizen of a city can sue the Mayor and city council if they actively engage in protecting illegal aliens. Looks great but you probably want to be careful of this in case you don't happen to have more money guns, and lawyers than the city you are filing suit against lest you find out that everything you own from your house and car to the brand of shoes you're wearing is in violation of city code and subject to impoundment.
Private persons and entities may initiate civil suits to obtain injunctions and treble damages against enterprises that conspire to or actually violate federal alien smuggling, harboring, or document fraud statutes, under the Racketeer-Influenced and Corrupt Organizations (RICO). The pattern of racketeering activity is defined as commission of two or more of the listed crimes. A RICO enterprise can be any individual legal entity, or a group of individuals who are not a legal entity but are associated in fact, and can include nonprofit associations.
The American Patrol link quotes the statute in some detail and makes for some interesting reading.