How do you calculate the group delay in samples of a Biquad Filter at a given frequency from the coefficients? If I have a BiQuad all-pass filter with a given set of coefficients such as:

g: 0.903228
a1: -1.88142
a2: 0.903228
B1: -2.08299
B2: 1.10714


And my operation per sample to process the filter is:

    inputGained_2 = inputGained_1;
    inputGained_1 = inputGained;
    inputGained = input * g;
    output_2 = output_1;
    output_1 = output;

    output = inputGained + (B1 * inputGained_1) + (B2 * inputGained_2) - (a1 * output_1) - (a2 * output_2);

    return output;


And this is running at 44.1  kHz: What equation could I used to calculate the group delay at a given frequency like say 82 hz?

Thanks.
Pick two different frequencies slightly above and below your target frequency. The choice depends on what numerical precision you have available and what your sample rate is. 
Calculate the z-transform at both frequencies
Calculate the phase at both points, make sure to check for "phase wrapping" 
Subtract the phases and divide by the frequency difference
Scale properly and multiply with $-1$