A user is essentially a second account. It has its own login credentials and features offered with its plan type. As such, adding a second user is the same cost as opening a second account.
To set up a user, go to your Add Addresses page, and add the address you'd like to use with this user. Then, click add it to A New Account. You can select what plan type you'd like the account to have, whether it should be on the same billing as your account or have its own separate billing, add its forwarding destination, and set up the password. You can also tick off whether or not this should be an admin account.
Each plan type has a limit on the number of total aliases that can be added, so if you need more aliases than your plan type allows, you can create a second user, and add further aliases onto this second user account.