I created an account by invitation to a private instance. I would like to delete that account. I don’t have a straightforward way of contacting the person who invited me. How do I delete my account (or request account deletion from the admin) through the Nextcloud application?
Contact the admin of the cloud that he deletes your account.
The admin himself/herself can choose if he/she uses the application that deletes users. But I guess it´s not intalled 
So the other easiest way is to contact him directly