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Molar absorptivity is completely independent of concentration of a substance as Molar absorptivity is represented by epsilon and is a constant. Absorbance of light is what is dependent upon concentration and will go down as concentration goes down and increase as concentration increases.

It is added to protonate the sodium benzoate.

Beer's law says that absorbance of a molecule or solution is:
A = a*b*c
where A is the absorbance, "a" is the absorptivity (in units of per molar per cm, M-1 cm-1), "b" is the path length (in units of centimeters, cm), and "c" is the concentration (in units of molar, M). The absorptivity, is also commonly known as epsilon.
That means that the absorbance is linearly proportional to the thickness of the sample, the concentration of the absorbing medium, and the absorptivity, which is a measure of a given molecule's ability of absorb light.
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The molar absorptivity of crystal violet can be determined using Beer's Law. Beer's Law is A=E*c*l where A is absorbance, E is the molar absorptivity, C is the concentration of the crystal violet, and l is the path length. Path length is how long the light has to travel through the solution. If you can find the absorbance of a certain concentration by using a spectrophotometer, where the path length is 1 cm, then you know all the variables and are able to solve for the molar absorptivity.

For example, the measured absorbance of 2.5x10^-5 M CV (crystal violet) is 1.64 with a path length of 1 cm. This means

1.64=E*(2.5x10^-5)*1

E=1.64/(2.5x10^-5)

E=65600

Happy Chemistry!