If I understood the need, I think you can do the following:

• Associating the event receiver to the content type (below I'll show you how)
• Get a reference to the subsite using SPWeb web = properties.OpenWeb();
• Perhaps you create a list in the root site to store the relationship between the group name and the subsite

Example of how to associate the event to a content type

public static void addEventContentType(string clase, SPContentType tipoContenido, string ensamblado, SPEventReceiverType eventReceiverType, SPEventReceiverSynchronization sincro)
{
 SPEventReceiverDefinition eventReceiver = tipoContenido.EventReceivers.Add();
 eventReceiver.Synchronization = sincro;
 eventReceiver.Type = eventReceiverType;
 eventReceiver.Assembly = ensamblado;
 eventReceiver.Class = clase;
 eventReceiver.HostType = SPEventHostType.ContentType;
 eventReceiver.Update();
}

If you need more information, please let me know.

If I understood the need, I think you can do the following:

• Associating the event receiver to the content type (below I'll show you how)
• Get a reference to the subsite using SPWeb web = properties.OpenWeb();
• Perhaps you create a list in the root site to store the relationship between the group name and the subsite

Example of how to associate the event to a content type

public static void addEventContentType(string clase, SPContentType tipoContenido, string ensamblado, SPEventReceiverType eventReceiverType, SPEventReceiverSynchronization sincro)
{
 SPEventReceiverDefinition eventReceiver = tipoContenido.EventReceivers.Add();
 eventReceiver.Synchronization = sincro;
 eventReceiver.Type = eventReceiverType;
 eventReceiver.Assembly = ensamblado;
 eventReceiver.Class = clase;
 eventReceiver.HostType = SPEventHostType.ContentType;
 eventReceiver.Update();
}

Sample use:

public static void asociarEventosGovernance(SPSite sitio)
{
 SPWeb mi_web = sitio.RootWeb;
 string ensambladoGovernance = typeof(EventReceiverGovernance).Assembly.FullName;
 string claseGovernance = typeof(EventReceiverGovernance).FullName;
 SPContentType tipoContenidoGovernance = mi_web.ContentTypes["Governance"];
 Internal.addEventContentType(claseGovernance, tipoContenidoGovernance, ensambladoGovernance, SPEventReceiverType.ItemAdded, SPEventReceiverSynchronization.Synchronous);
}

If you need more information, please let me know.