I accidentally turned off Family Sharing and left family group, what to do?

Ask to be invited to rejoin the group, or if you are the Organizer make a new group.

Click on this link to learn how to set up Family Sharing --> Set up Family Sharing - Apple Support

Read the section: "Invite people to join your family"

and at the very bottom:

"Remember, you can only join one family at a time, and you can only switch to a different family group once per year."