How to add all users to sharepoint standard group members

Question

Can I add "All" (Users) to SharePoint group members?

string userGroupName = "Member";

SPSecurity.RunWithElevatedPrivileges(delegate
{
    //Don't use context to create the spSite object since it won't create the object with elevated privileges but with the privileges of the user who execute the this code, which may casues an exception
    using (SPSite spSite = SPContext.Current.Web.Site)
    {
        using (SPWeb spWeb = spSite.OpenWeb())
        {
            try
            {
                //Allow updating of some sharepoint lists, (here spUsers, spGroups etc...)
                spWeb.AllowUnsafeUpdates = true;

                SPUser allusers = spWeb.EnsureUser("NT AUTHORITY\\authenticated users");
                if (allusers != null)
                {
                    SPGroup spGroup = spWeb.Groups[userGroupName];

                    if (spGroup != null)
                        spGroup.AddUser(allusers);
                }
            }
            catch (Exception ex)
            {
                //Error handling logic should go here
            }
            finally
            {
                spWeb.AllowUnsafeUpdates = false;
            }
        }
    }
}

UPDATE 15:28

string usersAll = "NT AUTHORITY\authenticated users";
SPSecurity.RunWithElevatedPrivileges(delegate
{
    //Don't use context to create the spSite object since it won't create the object with elevated privileges but with the privileges of the user who execute the this code, which may casues an exception
    using (SPSite spSite = SPContext.Current.Web.Site)
    {
        using (SPWeb spWeb = spSite.OpenWeb())
        {
            try
            {
                spWeb.AllowUnsafeUpdates = true;
                SPUser allusers = spWeb.EnsureUser(usersAll);
                if (allusers != null)
                {
                    SPGroup spGroup = spWeb.AssociatedVisitorGroup;
                    if (spGroup != null)
                        spGroup.AddUser(allusers);
                }
            }
            catch (Exception)
            {
                //Error handling logic should go here
            }
            finally
            {
                spWeb.AllowUnsafeUpdates = false;
            }
        }
    }
});

Answer 1

Yes you can add All Authenticated Users or Domain Users.

This will provide permission to all Users in Active Directory.

// "c:0(.s|true" = All authenticated users
web.SiteGroups["Member"].AddUser("c:0(.s|true", string.Empty, 
 string.Empty, string.Empty);

Update

You can get Web's read permission group using following property

spWeb.AssociatedVisitorGroup

Answer 2

I'm using the following script to add all Domain Users of my domain to the SharePoint Group "Members" for a given site. Note that Claims Authentication is used in this scenario.

function Add-DomainUsers([string]$SiteUrl)
{
    $spWeb = Get-SPWeb $SiteUrl
    $account = New-Object System.Security.Principal.NTAccount("MyDomain\Domain Users")
    $strSID = $account.Translate([System.Security.Principal.SecurityIdentifier])
    $spWeb.SiteGroups["Members"].AddUser("c:0+.w|$($strSID.Value)", "", "", "")
    $spWeb.Dispose()
}