There is a difference between needing multiple email addresses and needing multiple users.
I need multiple email addresses
An email address will deliver mail to the inbox of your Fastmail account, without needing to log in under a different username. You can have up to 600 email addresses in your account for no extra cost.
I need multiple email users
A new user will have its own login credentials, its own mailbox, and its own storage quota. Users are helpful if you have other folks in your family or business who need an email account, and you want to pay for all accounts at once.
If this suits your needs, you will want to add new users.