There is Users WCF services that you can use to get the data from one of the websites. Of you can create your own service to communicate between the websites.
You can view Users' service help by typing "http://host/YourProject/Sitefinity/Services/Security/Users.svc/help
". This will show you all available methods that you can use out of the box.
For instance, to get the first 20 users from the default provider you can call
First to get access to the services, you should authenticate a user. It is better to use an admin user, so you can operate with the data properly. Also if you are going to make requests on the same domain you can easily share the authentication cookie, otherwise you need to create a shared repository that you use to get and set the cookie to the response.
You must perform a GET request to the project A - from where you pull users' information from and send it to project B where you will import the custom properties. Using our service you will receive XML or JSON.
Basically to get the data through the service is easily than saving it. This is why you can use our service only to get the information you need like we do in the migration tool(Sitefinity migration tool
that upgrades 3.7 project to 4.0 projects by invoking a service). If you are going to use our service to update a user or create a new one you need to make a web request and using client side System.Net.WebRequest() method.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items