ow_system_plugins/base/views/controllers

I asked about .html, not .php, I mean how can I edit structure of join page? 

I wanna make a different page for guests, like on facebook.

oh damn, I guess u didnt understand.

then I'll ask something else.

I found in join.php this line:
$this->setTemplate(OW::getPluginManager()->getPlugin('base')->getCtrlViewDir().'join_index.html');

and now my question is, where is join_index.html located?

Or how can I set another template for join.php? I mean, create a simple .html page and set join.php to use it.

ow_system_plugins/base/views/controllers

All .html files for system pages, including join page, are located under"Views" folder.

I deleted all code that was inside join_index.html, I refresh join page on my site and nothing happend, it still shows the same register page as the default one ( like I didn't delete something ).

Enable DEV mode and check your site again.

you are good supporter, dear Aliia

my site (QQ) is based on lovely oxwall

QQ) is thankfull from you too. :)