If you are writing a thank you, you need to call the people who signed and ask for their address. If that is not possible, call the person whose address you have and get the other addresses or phone numbers. There must be somewhere you can contact the people whose addresses you need. Individual responses are more personal and appreciated.
If this is work related and calls for a formal response, individual responses are still called for. However, you should use their business address if it is business related. Be sure to put cc at the bottom so they are aware that each receive her own copy.
More simply, I would write back to the person for whom you have an address, and say "I would like to thank you and xxxx and yyyy for your ......" and end by saying, "Please pass on my thanks to the others on my behalf".