When I ran the following command (with our data), I got the average beta values for each segmentation i.e. only 5 beta values (as opposed to 5 beta values for each subject).

mri_segstats --seg ocn.mgh --i beta.mgh --excludeid 0 --avgwf 
beta.clusters.dat

That said, is it necessarily to run the previous command for each subject  (in order to extract 5 beta values for each subject) or is there way to extract betas for each subject from the group analysis?