Is there any way to open up an API through http-functions that will allow me to have a “master” site and some other sites for specific items but if the user registers on one of the sites it creates the membership on all the other sites as well, so they can login without having to create a new account for each site?
Send the registration data from the parent site (i.e. email, password, name) using Fetch to the child sites.
Expose the api of the child sites via https://www.wix.com/corvid/reference//wix-http-functions.html to receive incoming requests and then use Wix Users Registration function on the backend to register the user.