OK, now i did this solution:

    List<String> groupsOfPerson = new List<String>();

    var groups = Umbraco.TypedContentAtRoot().First().Descendants("Group");

    foreach(var group in groups)
    {
        foreach(var memberId in group.GetPropertyValue("Members").ToString().Split(','))
        {
            if(memberId == CurrentPage.Id.ToString())
            {
                groupsOfPerson.Add(group.Name);
            }
        }
    }

But I'm not very satisfied ...

Has anyone a better solution for this?

Greets, Silvan