When to ask invited users to register?

1. Why do you need user registration?
2. What is registered username?

Why do you need user registration?

Fraud prevention and data protection

This means you can easily check when an entry was altered, who deleted a category and if any exports have been produced. You can also easily mitigate the risk of voter abuse (i.e. one unknown user voting thousands of times).

What is registered username?

A registered user is a user of a website, program, or other systems who has previously registered. Registered users normally provide some sort of credentials (such as a username or e-mail address, and a password) to the system in order to prove their identity: this is known as logging in.