Is I want to invite you all correct English?

You need a little more context to be sure. Say that I send an email to my 20 closest friends ever, and describe a celebration that I am having. At the end I might say "I want to invite you all!" That is fine. Better would be: "I am inviting all of you to come! Be here by 7pm so we don't spoil the surprise. RSVP by Wednesday." It is understood that verbs in the second person (you sing, you play, you eat) do not distinguish between ''one'' other and ''more than one'' other; you is singular and plural depending on context. This is just a characteristic of our English language that is understood. If the context is clear, then you don't have to say 'you all'. It is perfectly fine to use colloquial terms like you all or y'all, but in more standard usage, make clear who the audience is and avoid the 'all' where possible.