You cannot create an account or sign up via web app. But let's put that aside.
You can ONLY sign up / register / add one device.
Not "at a time", even. At all.
You use your tablet, your phone disconnects.
You use your computer, your laptop disconnects.
You switch from laptop to desktop a few times, and now it's suspicious and gives you additional difficulties or bans you outright (that may or may not still be the case, but was when I tried this nightmare of an app a year ago or so).
You can only have ONE mobile and ONE computer device, at best, if you're lucky, and stars align, and it's second Tuesday of the month and the moon is full and you sing praises to Garmunklee the demon of impractical communication methods. Trying to access it on work and personal laptop; work and personal phone; or on phone and tablet; all is completely outside the supported use case and is for all intents and purposes impossible.
Compare to old-school ICQ, AIM, any XMPP, or Hangouts or indeed email etc, where you can seamlessly transition communication from device to device - and even check your messages on trusted public or 3rd party device such as friend's computer - and it's like waking up in a twilight zone.
It is pretty much the most architecturally user-hostile system I've encountered in my life, but again - I'm clearly in 2,000,000,000 vs 1 minority here :D