Filemail Business accounts can have multiple users. Administrators can add more user accounts by going to Company Settings.
Each user with a role other than AdminOnly requires a spare license (AdminOnly role is an administrative type of user - who cannot send/receive files - that's why it doesn't need a license). When adding users without having a spare license Filemail will prompt you about that fact and will add a license automatically.
You can find information about your active user licenses in the Account section.