<SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel := True)> _
<SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel := True)> _
Public Class SPSiteCollection _
    Inherits SPBaseCollection _
    Implements IEnumerable(Of SPSite), ICollection,  _
    IEnumerable

Dim instance As SPSiteCollection

[SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel = true)]
[SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel = true)]
public class SPSiteCollection : SPBaseCollection, 
    IEnumerable<SPSite>, ICollection, IEnumerable

Dim webApp As SPWebApplication = SPContext.Current.Site.WebApplication
Dim siteCollections As SPSiteCollection = webApp.Sites

Dim siteCollection As SPSite

For Each siteCollection In  siteCollections
    Dim changes As SPChangeCollection = siteCollection.GetChanges()
    Dim change As SPChange

    For Each change In  changes
        If change.ChangeType = SPChangeType.MemberAdd Then
            Dim webSite As SPWeb = siteCollection.OpenWeb()
            Dim groups As SPGroupCollection = webSite.Groups
            Dim list As SPList = webSite.GetList("Lists/Announcements")
            Dim items As SPListItemCollection = list.Items
            Dim group As SPChangeGroup = CType(change, SPChangeGroup)

            Dim item As SPListItem = items.Add()
            item("Title") = "User added to " + 
                groups.GetByID(group.Id).Name + " on " + 
                change.Time.ToString()
            item.Update()
        End If
    Next change
Next siteCollection

SPWebApplication webApp = SPContext.Current.Site.WebApplication;
SPSiteCollection siteCollections = webApp.Sites;

foreach (SPSite siteCollection in siteCollections)
{
    SPChangeCollection changes = siteCollection.GetChanges();

    foreach (SPChange change in changes)
    {
        if (change.ChangeType == SPChangeType.MemberAdd)
        {
            SPWeb webSite = siteCollection.OpenWeb();
            SPGroupCollection groups = webSite.Groups;
            SPList list = webSite.GetList("Lists/Announcements");
            SPListItemCollection items = list.Items;

            SPChangeGroup group = (SPChangeGroup)change;

            SPListItem item = items.Add();
                    item["Title"] = "User added to " + 
                    groups.GetByID(group.Id).Name + " on " +
                     change.Time.ToString();
            item.Update();
        }
    }
}