I currently have Signal set up in all my user profiles on GrapheneOS. I use Molly though so slightly different, but I think its needed for this to work. here’s what i did:

Molly is set up in my owner profile. open the app, go to settings, link a device, select the camera option so you can scan a QR code. This just speeds up the process of linking, you’ll see later. Exit the app and switch profiles. Download Molly onto desired secondary profile, if its already installed just proceed reading. Make sure “allow running in background” is allowed for that user profile and that Molly has unrestricted battery access on both profiles before you try to link accounts. On secondary profile, open Molly and tell it to link a device. Tell it to use QR code to link accounts. You should still be on secondary profile at this point.

With another device, take a photo of the QR code that was generated from your secondary profile. Right after that, as quickly as you can, switch to your owner profile and scan the photo of the QR code that you took a photo of. The QR code expires after a certain time after its generated, so the faster you can scan, the better. Since we opened the camera for QR scanning before switching profiles, as soon as you open Molly on your owner profile, the app will open straight into the camera. It might fail to scan the first time. I scanned the QR code repeatedly until it asked me to confirm linking the account, to which you can say yes. It can only port over the last 45 days worth of messages so don’t expect to have it be a mirror copy of your owner profile. But any messages from then on out will save and will be so much more convenient instead of switching user profiles to respond to a text.

As others have mentioned, local backups wont work across profiles, so be weary of that. I think backups will only work on your owner profile. I would just re-link my profiles again and accept that Im going to lose old messages on my secondary profiles if I needed to use one of my local signal backups. Like I said this is just a more convenient method of being able to use signal across profiles. Like for me, I have a completely isolated user profile that has google maps and android auto installed, and before i figured this out, i couldnt respond to signal messages or calls while on a long road trip for example, which was inconvenient beyond belief especially when trying to plan things with my group while navigating. Yes I would get cross-profile notifs so I knew when I was being reached, but I dont have to switch profiles and interrupt my map directions anymore when I respond.

Also, enable “exploit protection compatibility mode” on all instances of Molly to save on battery. Before I enabled this, Molly was using so much battery that I could have a screen time of less than an hour all day, and still hit a low battery notification every day. I’m talking, when I look at my battery stats by the hour, Molly will have consumed 39 percent of my battery for that hour, and after enabling the compatibility mode, its down between 1 and 5 percent and my phone actually lasts all day again. I wanted to bring this up because especially if you’re running Molly on multiple profiles, if you let those user profiles run in background, your battery life will suffer majorly unless you have exploit protections on.

Hope this helps. If anyone has critiques for my setup let me know. Besides the obvious that I probably have slightly less security or whatever

Edit: added extra detail