I accidentally left family sharing. How do I add it back?

See this article Set up Family Sharing - Apple Support

Join a family group

Accept or decline an invitation to join a family directly from your device. You'll get an invitation sent to you by email or in a text message. Then you can respond right from the invitation. If you miss the email or text invitation, no problem. You can respond from your device Settings or System Preferences:

• On your iPhone, iPad, or iPod touch, go to Settings > [your name] > Invitations. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Invitations.
• On your Mac, choose Apple menu > System Preferences > iCloud > Manage Family.

To accept an invitation, follow the onscreen instructions. When you join a family, you might be asked to confirm your account information and to opt-in to any features or services your family set up.

If you can't accept the invitation, see if someone else joined a family with your Apple ID or is sharing purchased content from your Apple ID. Remember, you can only join one family at a time, and you can only switch to a different family group once per year.

You will have to get an invitation from the organizer.