You can add a teammate in your profile in your personal account. Your account will be considered primary. You just simply need to specify the mail of the partner you want to add.
After you have provided your teammate's email address, they will receive an email. There he can follow the link, fill in the required data and automatically add to your team.
You can assign a specific website to a teammate, or a teammate can add a new website themselves. Main account sees all the websites that their teammates has added, while teammate sees only the websites they add or were assigned to.
If necessary you can delete your teammate by clicking on the "trash bin" icon in your profile. After a teammate removal all the websites added by teammate will be removed from dashboard.