The only problem is that you cannot rename the trust_level_group_0 https://meta.discourse.org/t/is-it-possible-to-rename-the-trust-level-0-group-to-something-else/96639.
But you can set up a hook in discourse that pings an endpoint, which in turn tells the discourse api to add a user to a group, on sign up.
Not pretty but it would work 