If you have Admin permissions you have the ability to add a new user.
On the left hand menu select "Settings"
Select "Users"
In the top right hand corner select "Add New User"
Complete user details
Assign the sites this user should have access to and set their home site (the site they'll automatically view when they first login)
Set their default User Permissions (find out more about permissions here)
Select "Send Invite"
This will trigger an email to the user prompting them to set a password and get logged in.
If you need to add a new user, but you only have manager/employee permissions, please contact your internal systems manager to assist in creating a new user for you.