Assign permissions for this group section was invisible after fabrik 3.0 installation.
how to fix
http://imageshack.us/photo/my-images/829/errorjf.png/

I've got exactly the same problem. Did you manage to resolve the issue ?

I could not find a solution :(

I thought I found a solution, and then realized that I did not re-enable every fabrik plugin. It would be great to get a solution to the problem.

Obviously, something with the fabrik plugin is preventing the k2 user groups manage screen from assigning permisssions. Probably a duplicate naming problem. I can see that the world element might cross over, as well as a couple of others.

I found a solution;

....\administrator\components\com_k2\views\usergroup\tmpl\default.php

42. <?php foreach($this->form->getFieldset('user-permissions') as $field): ?>

find and change
<?php foreach($this->form->getFieldset() as $field): ?>

is work