#create a file in the /Extensions directory called member_csv.py
def write_csv(self, file='/tmp/memberdata.csv'):
    ''' writes the memberdata of a portal to a csv ascii file '''
    props = []
    membership = getattr(self, 'portal_membership')

    for p in dir(self.portal_memberdata):
        if p[:1]!='_': props.append(p)

    fd = open(file, 'w')
    props.insert(0, 'id')
    for prop in props:
        fd.write(prop + ',')
    fd.write ('\n')

    for id in membership.listMemberIds():
        for prop in props:
            user = membership.getMemberById(id)
            if hasattr(user, prop): 
                fd.write( str(getattr(user, prop)) + ',')
            else:
                write (' ,')
        fd.write('\n')

    fd.close()

#create a External Method inside the CMF portal called exportDataToCSV
#Id: exportDataToCSV
#module name: member_csv
#function name: write_csv

#create a Script (Python) called exportMemberData
exportDataToCSV(context)
return 'exported successfully'

#you can just 'Test' your exportMemberData Script for it to execute #the External Method.  It creates a file in /tmp called memberdata.csv