You need to provide some more information in your question. Do you want to mean guest user to Admin in account? Though RingCentral has provision to add users and assign roles in the account but you need to be more specific in your need in the question with more details

I was purchasing another business. I had the current owners listed as guest so we could communicate and complete task among eachother. Now that they are guest I am unable to set them up as users due to the unique email erroring out each time. It says this email has already been used.

Hello @Dan Avon, we need to coordinate it with our App Support. Please send us an email to community.support@ringcentral.com and provide all email addresses. We'll check it for you. Thank you!