Thank you for your all help.
Thank you for all your help.
I don't know which one to use in the forums whenever I want to thank a member.
Which one is correct?
You don't need to use either of them. Simply click the "Like" button on a post you found helpful. It is quite time-consuming for us to keep clicking on a thread thinking that there is a new post containing a follow-up question or some new information, only to find that it just says "Thank you" in one form or another. The "Like" button really is enough.
Outside of this forum, as Rover said, your second suggestion is correct. If more than one person helped, you can say "Thank you all for your help".
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.
I always use "like" button.