The number of users you can add is unlimited. Each user, above three, is charged an additional $25/month with discounts for ten users or more.
Go to: Admin->Users and fill in each user you want to add. We 'highly' recommend you take your time in adding users until your company gets familiar with PixtaFlow. As you may be finding, it takes time to learn the system and rather than having 15 users trying to figure it out all at once, having one person who is familiar PixtaFlow within the company will save lots of time and headaches. That said you can set up as many users as you need using the menu item mentioned above.