conflating the specific binary instantiation with the general cryptosystem. Regardless, depending on your threat model, you can take increasingly { reasonable | paranoid } precautions like manually compiling and loading Signal, as it's OSS.

edit: "private group" can encompass a lot, especially in other ecosystems like Google and FB. If said "private group" adversary is, say, a prominent and wealthy Silicon Valley businessman and enterprising vampire who collaborates with fascists, then you can see the potential of compromising someone's security by coercing Google or Facebook engineers to run you a Hadoop query or conditionally inject malicious JS.