You can't sign an event on behalf of someone else, and the list of people you follow is an event like any other on nostr.

Anyone can add your public key to that list and sign it with their secret key, and there's nothing anyone can do to remove it. They'll see your notes because they're available to everyone.

What is possible is running a private relay that you post your notes to and only allowing certain people to access it. If you're notes aren't publicly accessible, you're more in control of who sees them. This is what I believe nostr.wine does with Creatr, which is like Patreon for Nostr.