Your cheapest option for a usable radio is going to be baofeng or talkpod. You will need some level of coordination ahead of time to get frequencies and tones right among the people you want to talk to. Have a backup channels plan and be sure everyone knows how to switch channels quickly from memory. Practice ahead of time.

You won't have encryption, so authorities might be listening too. Use code names for people and rally points.

It is possible with both of those too transmit on frequencies that are illegal or licensed only. In many countries committing crimes using radio communications has a compounding effect on your sentencing. Luckily these radios are $25 to $50 each. If you think you are about to get arrested, drop it and forget about it.