The GSG is a specialty group within the Association of American Geographers (AAG).  It costs only $5 (student $2) to join the Water Resources Specialty Group, but you must be a member of the AAG to be a member of WRSG.  To join the AAG, visit the AAG web page:
         Association of American Geographers (AAG)
and click on "Membership".  From there you can request the online form to join.

The cost of membership is graduated based on income.  As of 1/13/2000 it varied from $55 to $157 per year.  There are also special student rates for AAG membership.