Hi @James Owen,
To restore the SharePoint groups, you first need to make sure the default groups exist. Go to <site URL>/_layouts/15/groups.aspx to make sure the deleted groups are present. When the groups are present, you have to create the associations. Go to <site URL>/_layouts/15/permsetup.aspx and select the corresponding groups in the form.
Finally, press OK to create the associations. Now the deleted SharePoint groups of the site are restored.
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