Unable to add user to ZOHO Campaigns

Unable to add user to ZOHO Campaigns

Hi

I am unable to add a user to my ZOHO Campaign. It said that -  This email address has been already used in some other organization. So you cannot add/invite this user.

This user email address is apart of my business and is connect to my ZOHO CRM and Project Apps.

Thank-you in advance.

Regards
Jason