Add users to sites within network at registration?

I am looking for a plug-in or combination of plug-ins that will assign users to specific blog(s) within our multisite network at registration. Are there any solutions that maybe utilize invite codes or a dropdown menu of available sites when the user registers? Would even work if an admin had to register them on their behalf, as long we can specify the sites they should join upon activation.

Does anything like this exist? I am using Simple Import Users right now, but that plugin only adds the users on a site-by-site basis.