Take a look at the resource page in the USFIRST website. There are a bunch of pdfs and presentations you can use to tell people what FIRST is really about. Back in the days I've always asked for some sort of hand out for people who are not familiar with our program, now I got what I wished plus many more.
Click this link directly to go to the Volunteer resource center:
http://www.usfirst.org/4vol/resourcectr/index.html
Or, you can find it under the volunteer page:
http://www.usfirst.org/4vol/index.html
If there are any off season competitions near your team, you should definitely consider bringing your new students there. Nothing is as good as the real thing. All the noise, excitement, teams, and robots.
Or, at the very least, show them some competition footage. Soap 108's page is an excellent resource if you are looking for videos. I suggest showing the Championship finals.
http://www.soap108.com/2004/events/cmp/index.cfm
http://soap.circuitrunners.com/