Family Sharing

Depends on what the organiser used to invite you.

E-mail or iMessage, ....

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > Family Sharing.
2. Tap Add Member.
3. Enter your family member's name or email address and follow the onscreen instructions.
4. Choose whether you’d like to send an invitation via Messages or to invite them in person. Then follow the onscreen instructions.

I haven't used it but I would expect it to pop up as a notification.