No, you can use Knudge to manage all of your clients in the same way regardless of whether or not they register or install the Knudge app.  Clients that use the app will have access to a shared to-do-list, a calendar view, a record of all messages related to a task, and the ability to set their own preferences for when and how they wish to receive communications.  If a client elects to not log in or install the Knudge app, notifications and messages will simply be delivered via email. Clients will still be able to mark nudges complete.