Hello, sorry for the late response, I guess we were all a little busy with the sprint
Since noise levels typically vary between recordings, and since the noise covariance plays an integral role during source estimation, I would suggest you calculate the noise covariance and inverse for each participant individually, and average across all participants only at the very end. If you wish to calculate any statistics, e.g. contrast two experimental conditions, you will need those individual solutions anyway.
Hope this helps!