They do require you to sign up with at least one device that has a cellular ID.

There are four people in my family and only three phones. My wife, my one kid and I use signal religiously. When the youngest became of age to be able to handle typing and reading, we decided we would add him on. First I tried using his tablet to create the account. It straight up didn't work. I started screwing around with it and eventually loaded up my Google Voice account over there. I could get it SMS me once. But the account kept stopping. So I flipped it back over to my real phone and tried to sign him up from that with my Google Voice account. It kept commingling his account in my account together. Turns out underneath the sheets it's using the digital ID of my cell phone, whenever I would flip back to my account, his account would eventually stop working.

At some point we got him a cellular watch and I was able to use the SMS on that to make the account and it was fine.

I'd like to think they're only using that to make sure people don't make unlimited multiple spam accounts, but requiring a cellular IMEI like number to make a signal account is kind of extra creepy.