All of EngagamentHQ's feedback tools include an option in the settings, that enables you to "Allow Unverified Participation"

Essentially, unverified participation means that any community member can engage with a particular tool without having to register on the site. They are simply required to add an email and username.

Their interactions are still captured in reports and analytics and their data is accessible for the survey tool. In the survey reporting, they are noted as 'unverified' until such time they verify their account.

As part of this process, unverified participants are sent an email asking whether they want to verify and create an account with your site.

The email text is as follows:

Hi Screenname,
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​Thank you for posting a comment on "Sitename". 

Please take the next step and Sign Up here. [link to Register page]
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​Signing up will help us keep you informed about this project and others.
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​In addition, it will make it easier for you to log in next time you visit us.
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​By telling us a little bit about yourself, you will help us know that we are reaching the broader community.
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​Thank you
​
​The team at "Sitename"