Coupon codes only apply to subscription or paid for groups. You can tell a subscription based group by a coin badge next to the group name.
To enter your coupon code for a group, tap on the group.
Then tap on "Enter a coupon code".
Then simply enter your coupon code, wait for the green tick and hit "Join".
You'll be prompted to set up payment details if you haven't already.
Payment will start after your free trail period if there is one.
That's it, you're in!