I know we have a global tool for blocking friends from porting to us. This idea is a supplement for when I want to grant one or more SPECIFIC friends the ability to port to me.
Currently on our friends list we have a little icon that we can click and "fill" in. As far as I can tell it's only use is some type of favorite friends feature or something.
My suggestion is to allow this "fill" feature to be a individual override to the global option. If I'm setting up a Malistaire run and I want to give Corwin and Devin the ability to port to me and not the rest of my friends (for whatever reason) I could fill in thier icon and boom they can port to me.