[tcfhgj]

But if in a group only one person uses a compromised client, all communication is compromised, isn't it?

[hellojesus]

Potentially, yes. But that's where Signal's protocol helps. It includes plausible deniability.

Therefore even if your chat log is leaked by one member, it isn't possible to probe person A sent the message. If person B was the leaker, anyone that person B has ever communicated with on Signal could have sent the message appearing to be from person A.