An idea for maintaining security: One could setup and run their own IP telephone server and use encrypted IP phones to communicate amongst their group. Asterisk is an example of an open source telephony server that runs on Linux. The server must be connected to the internet and has to be made specifically aware of the particular encrypted phones that the group uses. The phones must also have Internet connections. The entire comm channel is encrypted. If the server can be kept secure, then I think there is no point in the channel that is subject to monitoring. If someone were able to piece together all of the IP packets for a particular conversation they would then have to break the encryption.
I have not proven this out in any way, but to my understanding this could be a solution. Keep up the good work. – Michael W.
Concerning the post, “Letter Re: Maintaining OPSEC  in a Geographically Extended Retreat Group,” one answer might by Skype . Video calls need not be used, calls can be made from computer to computer or computer to phone, and conference calling is possible. For some details on encryption see this article . With Very Kind Regards, – Suburban Survivalist