Hello,
There is currently no plugin doing this afaik. It used to be a possible workaround to share an invite link that would overrule the need for approval, but the security around that has been tightened, see Why do I need to still approve users that I have invited?
Might be worth putting your vote towards the feature request to change that:
Maybe take a look at the Custom Wizard Plugin 
and see if that can do anything.
And lastly, you could maybe have a setup with a custom user field + the Discourse Automation plugin, but that would be A) not secure because people could just lie on the user field + you’d still need to write a new script for the plugin.