I found out today all Discord invite links for this game have been cancelled.
More info on Discord Help channel pinned posts and the #posts -channel.
I use Discord app on my Ubuntu Linux, and have been able to log in just fine.
Invites are coming back once the current issues are resolved.
Meanwhile, if you have user account on Discord, you can log in at least on Discord App. I think it should work also on browser. (it does, just tested it)
If you do not have an account on Discord, my understanding is you can not join until the invitations come back.