Assign values to navigation property when adding new entity with BreezeJS - entity-framework

Question

I have two models:
public partial class User
{
public int UserID { get; set; }
public string FullName { get; set;}
public int GroupID { get; set; }
public virtual Group Group {get; set;}
}
public partial class Group
{
public int GroupID { get; set;}
public string GroupName { get; set;}
}
Whenever I create a new user in the UI, I will also assign the group to the user through the dropdown list. I will retrieve the group entity from the selected value in the dropdown and assign it to the navigation property when creating the user entity.
groupEntity = { GroupID: 1, GroupName: ABCD }; // group entity retrieved from dropdown
manager.createEntity('User' {
FullName: 'Boon',
Group: groupEntity
});
The following returns me an error.
An Entity cannot be attached to an entity in another EntityManager. One of the two entities must be detached first.
So I detached the entity from the breeze manager that retrieves it and it returns me another error when creating.
Uncaught Error: Cannot attach this entity because the EntityType (Group:#Data) and MetadataStore associated with this entity does not match this EntityManager's MetadataStore.
Entity Framework Code
readonly EFContextProvider<EFEntities> _context = new EFContextProvider<EFEntities>();
[HttpGet]
public string Metadata()
{
return _context.Metadata();
}
[HttpGet]
public IQueryable<Data.User> GetUsers()
{
return _context.Context.Users
.Include("Group");
}
[HttpPost]
public SaveResult SaveChanges(JObject user)
{
return _context.SaveChanges(user);
}
Any advice on how do I assign the navigation property to a preset values when adding a new entity?