The type ‘Microsoft.SharePoint.SPUserCollection’ has no constructors defined

I need to get users within SPFieldUserValueCollection and set them inside SPUserCollection, but I have the following build error:

The type ‘Microsoft.SharePoint.SPUserCollection’ has no constructors defined

 SPFieldUserValueCollection employees = item["employees"] as SPFieldUserValueCollection;             SPUserCollection users = new SPUserCollection();              if (values != null)             {                 foreach (SPFieldUserValue userValue in employees)                 {                     users.Add(userValue.User.LoginName, userValue.User.Email, userValue.User.Name, userValue.User.Notes);                 }                  return users;             } 

How I can initialize SPUserCollection?