I have a contact who uses Thunderbird and PGP. I added him to a group, and he can read the encrypted group messages. However, he cannot “reply to all” without turning off encryption because for some reason he doesn’t have the keys of all the group members. I think the best way to solve this problem would be to include the public keys of all the recipients (i.e. all the group members) in each message. I understand the Autocrypt-gossip header exists for this purpose.