R – Modify sharepoint view to group recursively (subgroup)

The first thing you need to know is that when you have a list with a User / Group field you must be aware of its type. When you have one user or group within the item value, the field type is SPFieldUserValue. However, if the field has multiple user / group selection the field type is SPFieldUserValueCollection.
I'll assume that your field allows a single user / group selection and you already has the following objects:

SPSite site;
SPWeb web;
SPListItem item;

Now, we'll check the field value for a user / group and retrieve a list of users, independant of which kind it is (the field's name is "Users").

SPFieldUserValue usersField = new SPFieldUserValue(mainWeb, item["Users"].ToString());
bool isUser = SPUtility.IsLoginValid(site, usersField.User.LoginName);
List<SPUser> users = new List<SPUser>();

if (isUser)
{
    // add a single user to the list
    users.Add(usersField.User);
}
else
{
    SPGroup group = web.Groups.GetByID(usersField.LookupId);

    foreach (SPUser user in group.Users)
    {
        // add all the group users to the list
        users.Add(user.User);
    }
}

I hope it helps you.

Tks,
Pedro José Batista

1. What you are doing here, is modifying the definition of the view for ALL users of the website, not only the current rendering instance of the page. This is why simple visitors cannot change it (they do not have such permission in the web)
2. If you want to do something, using the "SystemAccount" token, you have not only to do the "using SPSite, using SPWeb", but also find the list and the view using the "strong" SPWeb objects
3. Instead of modifying a view definition at runtime, you might want to consider using ListViewByQuery class http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.webcontrols.listviewbyquery.aspx