Along with the Time To Pet Client App comes some advanced Client App Reporting!
Time To Pet will "tag" clients that have downloaded and logged into the Client App directly on their client profile page. Just look for the purple "Has App" tag underneath their name.
You can also run reports directly from the Clients tab in Reporting. You can filter your clients by those that have downloaded and logged into the Client App and those that have not.
You can use this filter when sending out mass emails to your clients as well. So if you wanted to send an email to your customers who haven't downloaded the App yet --- it's very easy to do so!