To tag a user you must grab their pubkey:
* Search their username in the search bar at the top of the 🔍 Global Feed and click their profile
* Tap the 🔑 icon which will copy their pubkey to your clipboard
* Go back to your 🏠 Personal Feed and tap the blue + button to compose your Note
* Add @ direcly followed by the pubkey
(e.g., #[1])