The really cheap ghetto solution is to buy bales or rolls of rock wool, just leave them in the plastic, and stack them in the corners of the room. Ugly, smells a bit funny, and not everyone likes the idea of being in the proximity of bare rock wool, but it works.
More expensively you might get something like Auralex Promax stands and put them in the corners.
It kind of sounds like you might all be on the same side of the microphone, or at least have that option by moving where you stand or moving the microphone. In that case you could get one of the Reflexion Filters (e.g. the Fame Reflexion Screen Premium for a cheap option) to screen off half of the room. I've used one, and they work surprisingly well.