When inviting a new member to the organization, you can specify a department when you create the invitation. If you choose not to include a department up front, you can always add one once the person creates their Simpli5 profile.
When inviting new members individually (via the Add a Member option), the Department field has been added below those for the member's name and email address. See screenshot below.
When inviting multiple members by CSV upload (via the Add Multiple Members option), a department can be specified for any or all individuals. The field is optional in the CSV file, but it is included in the CSV template download (available at the Add Multiple Members screen).