Leaving a group
In the group, open the … menu in the top right, choose Manage group (or Group info if you are a member) and tap Leave group at the bottom. Your quotes stay in the group. You are no longer shown as a member of the group.
You can't leave a group if you are its only owner and there are other members. Make someone else an owner first.
Are you the last member? Then the group is deleted, together with all its quotes.
Deleting a group
Only an owner can delete a group. Are there still other members in the group? Remove them first in Manage group. Then tap Delete group at the bottom below Leave group. The group and all its quotes are then deleted. This can't be undone.
A deleted group is hidden from everyone straight away, and is permanently deleted after 30 days.