Newbie Qstn, Adding entities to the tree vs EntityManager.AddEntity

   As soon as you do:
  existingEnitityInstance.QuestionnaireInstanceMeta.PersonalDemographic = new PersonalDemographic();

you've added the new entity to an EM so there's no need to then call AddToManager.   To first create the entity (detached) and then implicitly add it you can do the following:

   var pd = new PersonalDemographic();
   pd.ID = Guid.NewGuid();
   existingEnitityInstance.QuestionnaireInstanceMeta.PersonalDemographic = pd;

AddToManager() is somewhat confusing - what manager?  If you construct an entity using CreateEntity<T> then you've tagged the entity to an EM, so the AddToManager() will add the detached entity to that EM.  If you construct an entity using a standard new constructor, the EM used will be the DefaultManager, which may not be what you want.

The method shown by smi-mark to create the entity and then call manager.AddEntity(entity) gives you better control over what's happening.

The EntityManager is not a global singleton.  The EM.DefaultManager is a global singleton but we caution against using it, and we'll likely deprecate it over time.  If you need a singleton it's best to implement your own.   See the Resource Center at http://drc.ideablade.com/xwiki/bin/view/Documentation/ for some examples (such as BadGolf) which show more robust patterns.