After you’ve composed your message, click the PGP Tray icon > Current Window > Encrypt, Sign or Encrypt & Sign. You can drag multiple users and their encryption keys into the Recipients list.
Alternatively, you can encrypt a file or message or multiple recipients in Kleopatra. Click Sign/Encrypt, then add recipients to the box next to Encrypt for others.
In both situations, PGP will encrypt the message so it can be decrypted by multiple recipients.