If you don't have the app installed yet, you can get it on the App Store or Google Play.
Step 2: Join your family
New Users
When you launch the app for the first time you'll need to create an account. Use the same email address that received your invitation and you'll be given the choice to join your family as soon as you're signed in.
Existing Users
Your family's invitation appears in Account ->Family. Each family has its own set of children and caregivers. Once you've set up your families, you'll be able to switch between the children from all of them.