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:

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.