* Uses the groups module to prepare a list of recipients and encrypt
  to those.
* The main version (encrypt-to-group.py) tries to check for invalid
  recipients, but still falls back to always trust for the second
  encryption attempt.
* The gullible version doesn't try pruning the recipient list at all,
  it just tries to encrypt and if it fails, switches straight to
  always trust.
* The trustno1 version doesn't use the always trust model at all and
  only attempts pruning the list of invalid recipients.