> The Commission has concluded herein that because uncompensated volunteer services are not considered to be a contribution under the Act, any individual, including a foreign national, may volunteer his or her uncompensated services to a candidate without making a contribution to that candidate.

From the PDF linked from here: http://www.fec.gov/pages/brochures/foreign.shtml#Volunteer

Whether the Sanders campaign actually wants foreign volunteers is a different issue.