If you click on Users on the left, you can easily add users.

Additional users are $50 per month.

There are 3 roles available to assign to each user:

At least one person needs the Administrator role. Click on a role to assign. Most users will have admin and salesperson roles or admin and project manager roles, but it's best not to have both salesperson and project manager roles.

The Salesperson and Project Manager roles have limited functionality, for example, they cannot access Company Settings and most of the menu items, and they may not be able to see deals that are not assigned to them.