By default nobody sees what you put in Notabula. You share deliberately, through a team.
Create a team
Go to Teams, click New team and give it a name. You're the owner.
Invite someone
Invite a team member with their email address. They receive an email with a link, create an account if needed and are then in the team. You choose a role for each member: admin, member or reader.
The team folder
Every team has a shared team folder in Files. Everything in it is visible to all team members; everything outside it stays yours. Team members can work in a document at the same time.
Handing over ownership or leaving a team
As the owner you can make another member the owner. If you leave a team, the team folder stays with the team.