//Find a random entity group that has at least one entity in it (in our real 
//   app, this logic is different but this is an easy way to reproduce the problem).
var aNonEmptyGroup = _entityManager.GetEntityGroups().First(g => g.Count() > 0);

using (var dummyStream = new MemoryStream())
{
    //Save the entities in the given group out to the stream
    _entityManager.CacheStateManager.SaveCacheState(aNonEmptyGroup, dummyStream);
}

//Go through the EntityAspects in the group and grab the corresponding Entity
//Is this the correct way to do it?  Will the .Entity property always have a value?
var entities = aNonEmptyGroup.Select(entityAspect => entityAspect.Entity);

//Save the entities in the given group out to the stream
_entityManager.CacheStateManager.SaveCacheState(entities, dummyStream);