shareWith method
//////////// //////////// Adds a user to the sharedWith list
Implementation
// II.E - Sharing Methods
///////////////
/// Adds a user to the sharedWith list
RaliRoute shareWith(String userId) {
if (sharedWith.contains(userId)) return this;
final newSharedWith = List<String>.from(sharedWith);
newSharedWith.add(userId);
return copyWith(sharedWith: newSharedWith);
}