you ahve to give them an id like,
<input type="checkbox" name="checkBoxGroupName[IDNUMBER] />
And then something along the lines of:
(taken from admin_reports.php in punbb)
if (isset($_POST['zap_id']))
{
confirm_referrer('admin_reports.php');
$zap_id = intval(key($_POST['zap_id']));
the key gets the number inside of the [] not sure how to exactly do it from more than one name at a time, tho you could get an idea of how to do it from admin_options.php as that uses the same idea i think.
Im at school atm and i just SSHed to my home computer to open up admin_reports.php to get that code, rest is from memory, but it should give you an idea of where to look.