It is a two step process:
First, go to the App Store on your iPad, and type in the search term "clubster app".
Next, tap on "iPad only" or "Filter" in top left of Screen and select "iPhone only".
You will then see the Clubster app appear for download.
Note, on iPads running iOS Version 11, you may need to use the search term "private clubs", instead of "clubster app", and then when the results appear, tap on the "iPad Only" or "Filter" in top left of Screen and select "iPhone only". You should see the Clubster app appear in the results.