New GDPR rules make this plug in very useful. However, it would be useful to also get opt in from anonymous users. I've created a branch to do it here. Are you interested in merging these changes or shall I fork the project?
I'm also thinking of adding new accounts to a group automatically within the extension. Also, having a special page to allow existing users to opt in/opt out of terms of service/privacy would be good. This is probably beyond the scope of this plugin?