What would be the difference between "how many people are invited" vs "how many people have been invited" vs "how many people were invited"?
Is the first option even grammatical? What are the differences between the three?
*I AM NOT A NATIVE OR TEACHER.*
Yes, the first form also is correct and meaningful.
When talking about an event regardless of its time, I'd use the first form: Google I/O is an annual conference. It is held by Google and thousands of developers are invited to discuss the future of Google technologies.
For the second form, we can imagine the same conference. Google has invited the guests but the conference has not yet finished or even started:
- Tomorrow is the fist/last day of Google I/O 2014.
- How many people have been invited to the conference?
* In this context, the third form also is acceptable to me: How many people were invited to the conference?
When talking about a finished event, I'd use the 3rd form: The conference was held by Google and thousands of people were invited to discuss...