Hi Mikal,
The police is using VHF equipment, and a famous brand for that is Motorola.
I didn’t understand what you wanted to do : youth talking in a device, and everybody else listening ?
As far as i know, the VHF equipments are half duplex (if someone talks, everybody else must shut up and listen).
That won’t work with 10,000 people, I fear. If you need to have people talking to few other people specifically + you want full duplex (everybody can talk at the same time), then you need more expensive equipment (GSM, for example)
So anyways, here is what you can check out in google / wikipedia:
VHF network
Ctizen Band (CB)
Police radio
I can help with general advice, but not specifically about the technics about such VHF networks.. Check Motorola website, maybe you can contact them for some support.
regards
pix