Group object

A protected block of the following structure:

{
    "participants": [ {
        "role": "owner" | "admin" | "writer" | "reader"
        "added_at": <datetime>,
        "party": <party block>
    }, // many participants
    ],
    "name": <string>,
    // - this section can not be described now as item references
    // are not ready
    // "icon_url": <item_reference>,
    //"background_url": <background_refereiced>
}