Whoever left the question for me on my meebo chat widget, you didn’t give me an email address for direct response to you, so I’m posting the answer here.
Unit Publicists who work on location during principal photography of union films and want to receive union benefits need to belong to the International Cinematographers Guild. Publicists and Cinematographers were merged into one guild in 2002. One of my co-authors, Michael Klastorin, belongs to the ICG. Incidentally, he hasn’t been around to post in what seems like forever because since the end of the writers’ strike, he’s been working almost non-stop on location.
Film publicists that don’t necessarily focus on unit work, rather in other areas of film publicity, such as launch, festival, theatrical release, etc., or who do unit work on non-union films or films with union waivers, don’t have to belong to a union. There are professional associations, however, that are useful to join. For instance, I belong to the Entertainment Publicists Professional Society.
Each organization has its own eligibility requirements, application and acceptance process.