Navigate to SETTINGS >> Admin Accounts. Click on "New Admin Account Button +"
On the Username field add the email of the person you are adding. On the Password field you can add your own, or you can generate a new password.
You can Copy, Fill Password or Generate another.
Under "Account Type" you can decide if the User you are adding is a Primary Admin (All Permissions) or a Moderator (Limited Permissions), this last one will hide the Revenue and Billing Information.
The "Account Status" you can use to temporarily disable the account, if the account is disabled then the User will not have access to the backend of the website.
Under "Contact Details" you can add their contact information.
Finally, you or the user can create the Email Signature.