Hi @stephen7,
Welcome to the Auth0 Community and sorry for the late reply!
Firstly, the user should be redirected to the Universal Login Password Reset page, not singup, since a user invitation is basically a change password link repurposed as an invitation
As mentioned in our documentation on the topic, the parameters that were set up above are correct. However to start the invitation acceptance transaction, it should forward both the invitation and organization parameters along with the end-user to your Auth0 /authorize endpoint.
There are also a few additional steps to be considered when doing this process from the application side, described under this Knowledge Article, which provides all the links and documentation needed on the matter.
Additionally, Organization Membership Invitations can be achieved via the Auth0 Dashboard or the Management API as well.
Hope this helps the case, but others as well.
Thanks,
Remus