When booking a class, you'll see the 'Bring a Friend' button on the booking confirmation popup that appears when you've booked a class. If you have enough Loyals Points, you can use them to get a code to share with a friend.
You can also generate a new Bring a Friend code without booking a class in the F45 Training app under Profile > Settings > Bring a Friend
Your friend will need to meet the eligibility requirements to redeem the code at your studio.