Here's how you invite employees to your account. There is also a guide on how to upgrade a user to administrator.
Here's how it's done:
Navigate to Employees.
Click "Invite".
Fill in their email address (you can also send the employee a welcome message).
Click "Invite".
We’ll send an email to the employee with instructions on how to sign up for your organization. In Employees the employee will get an "Invite sent" status, until his/her sign up is completed. If the employee doesn't receive this email within an hour or two, ask him/her to check the junk or spam folder or read reasons why emails get stuck.