your update code is not correct:
mysql_query("UPDATE users SET groupID = '2' WHERE $_SESSION = ['SecurityAssist_UserID']");
you need to either update on the update based on the UserID column and the value in the SecurityAssist_UserID session:
mysql_query("UPDATE users SET groupID = '2' WHERE UserID = $_SESSION['SecurityAssist_UserID']");
or the email column being equal to the $payer_email value, assuming the name of the email column is UserEmail::
mysql_query("UPDATE users SET groupID = '2' WHERE UserEmail = '$payer_email'");