I would like again to thank everyone who helped me.

Thank you!А без who нельзя?

I would like to thank again everyone, who helped me

может быть и можно, но коряво
https://www.google.com/search?q=thank+everyone+helped+me&ie=utf-8&oe=utf-8&client=firefox-b
обратите внимание - все результаты с who or that

Да,grumbler,согласен,все примеры с who.

Sergii Dermelov, а есть правило насчет места again в предложении?

again можно и перед to thank и после него?

можно

Thank you