If this is not on the home screen then tap the app drawer icon shown below and Play Store will be in there
In the Play Store, search for the Smartcode app.
You may get something that looks like the following, in which case tap the icon or title:
You'll be on a page that looks similar to:
Tap the Join button, and Join in the popup
and the text will change to:
The time this takes seems to be unpredictable. Sometimes it takes a few seconds, sometimes fifteen minutes.
The text doesn't auto update, you will need to tap back and come back into the app's detail page every minute or two.
Once you are entered into the beta, the text will become:
And the Open button at the top will become Update letting you update to the Smartcode (beta) version of the app.