This question is somewhat philosophical in nature. I cannot necessarily tell you HOW to get hundreds of people to stand with you in opposing the National Socialist Movement Rally in Austin on November 11th; however, I can offer you a list of suggested resources that might help put you in contact with like-minded people. First is a list of organizations, then some suggested books available at the Austin Public Library:

***Here are some organizations that you might want to contact:

Anti-Racist Action (Austin Chapter)

Anti-Racist Action posted an announcement to the Austin Independent Media Center about protesting this event

Independent Media Centers are often places activists/groups use for networking, so you might want to contact them directly for information.

Third Coast Activist Resource Center has a list of Austin activist groups on their website

United for Peace and Justice also maintains a list of Austin activist groups.
(You can also post notices/events on their website)

You might try posting a notice on Austin Craigslist (like in the volunteers or activities section) to see if you can recruit any interested volunteers:

***If you want some ideas on how to organize activist gatherings, you might like these books, which are all available in the Austin Public Library system (ask your librarian!)

- The Global Activist's Manual: Acting Locally to Transform the World
by Mike Prokosch and Laura Raymond

- Globalize liberation : how to uproot the system and build a better world
by David Solnit

- Organizing : a guide for grassroots leaders
by Si Kahn

- No more prisons
by William Upski Wimsatt

I hope this is helpful. Of course, whatever local organizations you are a part of (schools, churches, clubs, etc.) can also be places for you to network with fellow activists. In addition to making contacts in person, you can also post flyers around town in public places and local establishments asking for volunteers.