Dear Carolina,

Congrats on finishing the preprocessing, well done! Now comes the exciting time of generating hypothesis and running group analyses!

For doing group analyses take a look at http://www.fmrib.ox.ac.uk/fsl/feat5/inde...
You might also have a statistician in your group who might be able to assist you with the statistics involved in setting up group analyses. It might also be an option to look into some of the courses that FSL and AFNI are offering preceding or following the big conferences such as HBM and SfN (e.g., http://www.fmrib.ox.ac.uk/fslcourse/mont...).

For looking at the data, take a look at http://www.fmrib.ox.ac.uk/fsl/fslview/in...
That link should explain how you can load images in fslview and indeed overlay them on top of that subject's structural images or a brain atlas or standard space image (e.g., MNI152 brains). The same can be done with AFNI, mricron (downloadable from nitrc) and basically any image viewer capable of taking nifti images.

Hope this helps!
Maarten