Depending on the plan you have there will be a limit on the number of users you can have on the platform at any one time. The trial and Startup plan allow for 3 users, The premium plan has 5 and Enterprise will be preset by the user's needs and can have upto 100 users.
To add a user click on Settings > Users > Add new user
You will then need to fill in the user's details and the rights they have. Click here to learn more about the different levels of access a user can have.