Go to the Dashboard and select Sitewide Settings in the left navigation.

Activate the Access tab.

Click on Click here to modify other permissions in the Other Permissions box.

Manually added groups aren't listed by default; they have to be added first. Click on Add Group or User and select your group in the dialog.

We only activate Change Content on Page Type Default Pages and Perform Full Database Backups. Backups almost never hurt and updating default blocks for a page type can be quite powerful for an editor, but also a bit dangerous. If your editors aren't people who work carefully, you might want to skip that option.