You can add your team members by going to the 'Team members' tab:
You can then add a new team member by clicking on the blue '+ New Team member' button. After you fill in the information and click on the 'Save' button, your team member will receive an email with their login information. In case they cannot find their invitation email (not even in Spam folder), you can resend that email by clicking on a "Resend invitation email" button.
Regular user, project manager or administrator? How to choose team member roles?
Important thing to note: your team members should not create their own account, but follow the instructions in their invite email. You will not be able to merge accounts later.
In case your team member has already created their own account, take a look at this article.
Note: please go to 'Projects' to assign a team member to a given project. Otherwise you will not be able to delegate tasks to them and they will not be able to track time on projects that they are not assigned to.