I think for server, it is not possible to change the default group for new users to anything other than the 'confluence-users' group.
I could only find this related question/answer, which I don't think helps much, here: https://answers.atlassian.com/questions/36116638
Is there any reason why you can't do something like the following steps?:
Hi Jean Bausch,
My guess its not possible to restricte confluence user group. its a default group when sign up to confluence users add into confluence user group. if you don't want user in confluence user group then move other group.
User sign up options don't Allow people to sign up to create their account
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