This article provides information on Google recent policy update regarding Reviews Invitations.
In addition to Prohibited Content guidelines, Text Reviews are subject to the following additional requirements:
Don’t use reviews for advertising purposes. This includes, but is not limited to, posting email addresses, phone numbers, social media links, or links to other websites in your reviews.
Don’t discourage or prohibit negative reviews or selectively solicit positive reviews from customers.
Don’t include promotional or commercial content.
Don’t offer or accept money in exchange for reviews.
Don’t solicit reviews from customers in bulk.
For more information on Googles Policy please follow this link: Google's Format Specific Criteria