Add users of one multisite to another multisite

I have several multisite blog networks and would like to automatically make new users of one network subscribers of another network. I use the New Blog Defaults plugin for within the network, but would now like to go network to network. (I have two networks for different targeted users and now a third network with support, forum, etc for users of the first two networks, and more networks to added.) How can this be done? I'm sure there will also be security issues?