To join a guided activity go to the main menu on the mobile app, and press on the ACTIVITIES button

When you're there, you'll see a list of all the activities of the day you selected at the top, and you can press in any of them to see more info and join

On the list of activities you can see the date and time of the activity, the instructor, and the available spaces to join. Press on any of the payment methods available to pay for the activity and your place will be booked.