This is possible if you have purchased a 1, 3 or 24 month subscription. If you have purchased the app once in the past this feature is unfortunately not available.
You can share the app as follows: Go to settings in the menu ‘This leap’ > ‘My account’ > ‘Share your subscription and your baby’.
Here you can invite 1 person via an email address. With this you share your subscription, so the other person does not have to buy a subscription. Make sure you also share your baby with this person. This will synchronize everything you keep track of your baby in the app such as the diary, checking off signals, skills and games.