I have the same exact issue from this thread : https://help.zoho.com/portal/en/community/topic/can-not-add-users-in-zoho-campaigns

I cannot post a reply on the thread so I created a separate topic.

My e-mail is vcorbilla@gmail.com and my boss cannot send invite , it says that my e-mail is already with a different organization, I need help to remove my e-mail on that organization so my boss can re-send the invite. 

Thanks!