Hi Pthfnder,
Thanks for contacting Apple Support Communities!
You can mention someone in a group message but the same doesn't apply to Notes.
If you'd like to learn more about how to mention someone in iMessage, this article will guide you: Reply to specific messages on your iPhone, iPad, and iPod touch - Apple Support
"How to use mentions
- Open a Messages conversation.
- Type a contact's name, then tap the name when it appears. Or type @ followed by their name.
Depending on your contact's settings, a mention can notify them even if they've muted the conversation. To change this notification setting, go to Settings > Messages, then turn Notify Me on or off."
To learn more about the collaboration features in Notes, here's a handy resource for you: Share notes and collaborate on iPhone - Apple Support
Take care!