You can get the name by creating an instance of the class and using the IndexName
property.
string indexName = new Users_ByEmail().IndexName;
I agree it would be nice if the API had some overloads that didn't rely on string. Something like the following would be nice:
documentStore.DatabaasCommands.UpdateByIndex<Users_ByEmail>(...)
If you have a lot of these calls in your code, you could consider writing an extension method that would give you that.