[Freesurfer] Problem about the fMRI Analysis?

I get the correlation result between two condition of the functional data by using AFNI( I do not know how to do correlation by using freesurfer) And then I use the bbregister to aline the correlation result (produce by Afni) to the anatomical result produced by using freesurfer. Next I using mri_vol2surf to aline the each one's functional result to the fsaverage offer by freesurfer. My question is :1 Should I need to do mri_vol2vol next for each subject? 2 I have two group of subjects. I want to know if there is some correlation different between them? What should I do next? Thanks very much! Linda