I'm curious as to the use case for adding users to a realm and not an application? I myself am uncertain, but I'm curious as to why you asked.
Anyone with Sharing permissions can invite a new user to an application, and thus the realm. Being a registered user of any app in your realm places that user in the realm. I have a realm management tool that gets updated every 24 hours by a 'quickbot'. So, anyone I invite today will not show up in that realm management tool until the next day. This may not be the case with your realm however.
As far as I know, any new user you invite has to be directly invited to an application within your realm. I'm still learning new tricks in QB, so I can say this here and now, and someone will come behind me and say something else that sounds off-world.
For what it's worth...
What does this do? Just curious.