Xero & Retool - whole organisation

I followed this to get our Xero resource set up and all is working well (really useful instructions thanks!), but as it's a Custom Auth, I'm missing the option to be able to 'Share credentials between users'.

Has anyone had any luck creating a connection to the Xero API for their whole application? I'd like to be able to allow our team to create invoices without having their own Xero logins.