mikal,
well, 10 000 ppl talking together is impossible. Do you want to setup a kind of Twitter, with voice, rather than text ?
I think your 10 000 friends should use mobile phones and use special apps like twitter or a “voice” twitter through the 2.5G or 3G network. They’ll have to pay, but that’s going to be less than setting up a wireless network…
Otherwise, check out VHF networks. Please google it, as it sounds like this is what you’re looking for.