According to a community forum post by Signal's VP of Engineering, Jim O'Leary, Signal are officially testing username functionality. The feature has been in development for some time now, and it is finally ready for public testing.
"It’s no secret that we’ve been working on bringing Usernames to Signal for a while now," writes O'Leary. "After rounds of internal testing, we have hit the point where we think the community that powers these forums can help us test even further before public launch."
Until now, Signal has required users to connect with each other via their registered phone numbers. Unfortunately, users will still need to register a phone number to use the service, but this option to allow users to connect via usernames would allow them to hide these phone numbers, enabling more private communication.
Users can test this feature now via Signal Staging. To do so, you'll need a new build of the app, a new account (thankfully you can use the same number you are using in the Signal Production Environment), and you'll need contacts who are also in Staging as this is the only way it will work right now.
After you are all set up, navigate to Settings > Profile and Settings > Privacy > Phone Number to test creating usernames, sharing username links, and other related features.
Just remember, this is pre-beta software, so many features may not work. Notably, Staging comes with no uptime guarantees, so don't try to use it for mission-critical communications, and push notifications may not work well or even at all.
If you do end up testing Signal usernames, the team are really hoping for feedback about crashes and UX improvements, so don't forget to provide feedback accordingly.
You can find more detailed information regarding download links, how to get setup, and how to provide helpful feedback at the link below.