As immmr is still in Beta, please download the app via TestFlight. See steps below.
Firstly, Please make sure to download the Testflight app from the App Store.
Open invitation email from Testflight on your mobile device and tap the button View in TestFlight.
In Testflight on the top left, Redeem invitation & enter code sent in Testflight invitation email. (Skip if you already have Testflight).
Download immmr from Testflight and tap Open. Register…and start to #SayItWithimmmr.