How do you calculate the group delay in samples of a Biquad Filter at a given frequency from the coefficients?
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 $-1$