Can multiple people be in one Instance?

Yes, you and your staffs can be on your Instance to manage invoices. To add your staffs, follow these steps
1) Login to your dashboard
2) On your side menu, click on the administration dropdown and select "Manage your users"
3) Click on the "Create user" button at the top right corner
4) Fill in the user's details and save
5) They can login using the credentials you used