[akanevsky]

Fatal error: Call to undefined function: parsehide() in /home/ddfiles/public_html/includes/class_postbit.php(279) : eval()'d code on line 1

You need to copy and paste from the instructions.
There isn't "parsehide", but instead there is a "parseHide"... Case matters.

Will this check secondary usergroups as well?

Yes.

if I know well, the hide hack is coded to read the post count and then let the user read the hide text.
the thank you hack will not increase your post count, so they cant work together for the moment.

Not with the same tag. I could make it with a different tag, such as [hide-no-reply]... However, I don't see a point in doing that. It's too easy to click a button, who cares about it.