Advice from Tod Daniels is a gold standard. He is the go to guy for solid accurate advice that people can lean on to get their visa problems sorted without needing an agent. He's a godsend to foreigners in Thailand.
The offending group member should be the thrown out of the group and blocked
Your first challenge will be opening a Thai bank account to deposit the required funds, ฿800k to start a non-o application in Thailand. It is becoming increasingly difficult on a tourist visa or visa exemption. It's much easier if you, apply for your 90 day retirement non-o in/from London and open your Thai bank account off the back of that.
Get your long term non-o (not non-oa) visa in Thailand, if you can meet the financial requirements of 800k THB in a Thai bank account. No insurance requirements. Easily done cost 3900THB for the first 15 months.