You can always link a firm user to a case after it has been created.
Open the Case Details page for the case that you would like to add the firm user to. Click the Contacts & Staff button (pictured below). You will be presented with a list of all the contacts and firm users that are linked to the case.
Click the button labeled Add Staff
You will then be given 2 options (pictured below):
New Staff - Create a new Firm User and link them to the case.
Existing Staff - Link an existing Firm User to the case.
Batch Link Firm Users to Cases:
Open the Cases Dashboard by selecting the Cases tab at the top of your screen. Click the checkboxes associated with the case on the left hand side then select the Actions button (pictured below). You will be presented with a popup box that allows you to select the users who should be linked to the selected cases.
Click the blue button labeled Add Case Link
Pro Tip: To link a firm user to only cases related to a specific attorney, first filter your cases list by a Lead Attorney, then use the select all button to easily link users to only these cases.