The groupId parameter specifies the unique ID of the group for which you want to retrieve group items.

This parameter can only be used in a properly authorized request. Note: This parameter is intended exclusively for YouTube content partners that own and manage many different YouTube channels. The onBehalfOfContentOwner parameter indicates that the request's authorization credentials identify a YouTube user who is acting on behalf of the content owner specified in the parameter value. It allows content owners to authenticate once and get access to all their video and channel data, without having to provide authentication credentials for each individual channel. The account that the user authenticates with must be linked to the specified YouTube content owner.