Get checked checkbox values in codeigniter

I am new to codeigniter. I wanted to know how to get checkbox values which are checked .I went through various answers but couldnt solve.

I am displaying the values from database table as checkbox values.

My view code for checkbox :

                                   <?php foreach($g->result() as $grp)
                                    { ?>
                                    <input type="checkbox" name="mycheck[]" value="<?php  echo $grp->group_id; ?>" <?php echo set_checkbox('mycheck', '$grp->group_id;' ); ?> /> <?php  echo $grp->group_name; ?> 

I don not know how to get only the checked value(that is the group_id) in controller as well as in model to insert it into another tabel in database..

Please help.