Type of the group preview. Use 'group_picture' to set a group picture as large preview, 'thumbnail' to set a custom small jpeg image from 'preview' param as preview, 'media' to set a custom large image from 'media' param as large preview, 'style1' to set a isometric style for group invite link preview stub, 'style2' to set a flat style for group invite link preview stub, 'style3' to set a WA style for group invite link preview stub, or 'none' to not set a preview. If not set, the default is group_picture.