This is a tricky one and it has to do with security. IoT Open is a true multitenant system where every organization is isolated from each other. The only information you can get from a user belonging to another organization is that the email address is in use. Otherwise there might be scenarios where you could gain information about another organization.
Someone in the organization above need to set this up. So if you mail our support at firstname.lastname@example.org they can fix it. Include information about what user should be added to your organization. We might contact the other user or organization to verify.
It is also possible to move entire organizations so that the whole organization of the user ends up under your organization.