Quote from wrbtrader:
stevegee58 suggestion is excellent and easily to implement.
Here's two other suggestion.
1) Similar to steve's suggestion, use a private discussion forum that have private individual threads that you determine whom has access to it and whom does not have access to it. Next, when you're ready post the trade signal...you just post it without any need for encryption.
2) Send your trade signals via "encrypt email" via services like OpenPGP (e.g. Neomailbox or Countermail), Sendinc and there are many software (e.g. datamotion, voltage) out there that can be installed as add-ons to whatever email you're using. These software or add-ons contains dozens of security features including "email encryption".
Usually the best ones do it all like desktop, cloud storage base, mobile devices, mail services and gateways. You can even set it up to be notified when someone has actually opened the email, how long the email was open, how many times it was re-open, IP address location when email was open and many other things...
The more features the higher the cost.
Anyways, I think the easiest solutions is steve's suggestion or a private forum solution prior to getting into the "encrypt email" stuff. The latter is usually reserve for some serious stuff.