This kind of plugin doesn't exist.
In order to create Users you should leverage Resources["EntityServices"].CreateUser() service, once the user it's created you can aggregate that User to the desired organization with corresponding Organizations["organizationName"].AddMember() service. You can wrap both services on a custom made by you that does both.
You will have lots of security things to overpass to do this, but it's hard to explain here. Collection Permissions, User Permissions, Services Permissions, and all wrapped in a secure way...