Earlier quoted context omitted.
I'm still of the opinion that encrypted private group chats are an impossible UX problem (1:1 chat is fine). But if I were to trust anyone to find a way to do it properly, it would be Signal.
What makes it impossible? Naively I would think that if you have a secure 1:1 communication protocol, then you can send N*(1:1) secure messages to a group of N people. To solve the "fake group message" problem where an adversarial member of the group sends different messages to different members of the group, or delays the message to some members, the protocol could simply allow for a 2nd level "vouch" message to be…
IIUC this is what iMessage does (at least when Messages in iCloud or whatever it's called is disabled), except s/people/devices: say you have three devices and someone sends you one message, the message is encrypted once per recipient device, and three encrypted messages get sent. Whether it's 1 person with 3 devices, 3 people each with 1 device, or 2 persons with one having 2 devices and the other a single one becomes largely immaterial.