Not only do you have to be in a union but you have to be a still photographer. In some cases they let cameras on the set for continuity and wardrobe, but to take stills of the shoot you must be a still photographer.
Now if you are a still photographer and you’re not in the union, you can get your days toward getting into the union by working on non-union projects, but be sure to check with Contract Services to make sure you are saving things like pay stubs, etc. so your days will qualify.