I wonder if you are new to the SuiteCRM interface? the Calls view and Notes view are slightly different; in the way they handle contacts.
In Calls - the people you add show in sub-panels. And so you can add 1 or more people.
But in Notes -there is just 1 field, in the form at the top ‘Contact’ .
If you want to play in a sandbox - maybe get a login to the demo site: see the page footer at https:suitecrm.com -under the heading ‘Community’ - look for Login; and then Demo.