To add a new Team Member, first you need to have Admin permissions.
If you do, then click on "Account Settings", found in the lower-left corner of the screen, as shown in the picture below:
Next, in the upper-left corner of the screen, click on "Company", then select "Members" from the drop-down menu, as shown in the picture below:
You will be met with the following screen:
Here you can see current members, their roles.
Additionally, you can type in the email address of the person you wish to invite as a Team Member.
After they accept the invite, they will appear in the least of current members as User by default.
To give them Admin permission, click on a "Pen" icon to the right, and select the appropriate role: