Adding a new user to FinancialForce follows the same process as adding a new user to Salesforce.
Navigate to Setup>Users and click the "New User" button.
From here, you will be taken to a user input screen where you will fill in crucial information about the new user. Key information includes the user's email address, user license and profile. At the bottom of the screen, there is a checkbox labelled "Generate new password and notify user immediately". This checkbox should be ticked if the user is to be given access immediately, if you wish to delay giving access, come back and tick this box later. Click "Save" to keep any changes.
The user is now created but has not been given access to FinancialForce Accounting. To do this, you need to assign a licence to FinancialForce Accounting by navigating to Setup>Installed Packages> FinancialForce Accounting and selecting "Manage Licenses".
You can add or remove users to be able to grant them access to FFA functionality. You may also need to assign permission sets to a user or assign them to a permission set group, depending on the setup in your org.