READI Collect is a web site, so there is nothing to download from an app store. You can still add it to your home screen, where it opens full-screen and one tap away. This takes about a minute and is worth doing before you need it rather than at the roadside.
Steps 1 and 2
Sign in on your phone and READI Collect offers to install itself. There is nothing to buy and nothing to download.
Sign in on your phone → Install → Share → Add to Home Screen
Signed in on a phone, a bar appears along the bottom offering to install. The i beside it explains what installing does; Install starts it; the cross puts the bar away for a month. It never appears once the app is already on your home screen.
On an iPhone, use Safari's Share button, choose Add to Home Screen, then tap Add. On Android the browser offers a real install button and does it for you. Either way you end up with a READI Collect icon that opens full-screen, without the browser bars.
Step 3
Worth reading before you decide, because the honest answer is that it changes less than people expect.
Settings → Notifications
Push notifications are not switched on for READI Collect yet, and this page says so. When they are, Safari only allows them for web apps that have been added to the home screen, so having it installed is what will let them reach an iPhone at all. Until then, everything still lands in your notification inbox.
It is on your home screen. If you are a collector, Topic 9 is the job itself; if you are on a company roster, Topic 7 covers requesting a collection for yourself.