Users exclusively belong to Organizations; every User belongs to exactly one Organization.
But I also see a screenshot where, after login, the User has to choose an organization or to create a new one. It seems to me that you support Users and Organizations in a many-to-many relationship, is that correct?
At my work, we landed on the terminology of Users, Memberships, and Accounts to describe this (a User can have Memberships to multiple Accounts, an Account can have multiple Members, etc). As a result, you don't "delete a user", you "revoke a membership".