[Marco van Herwaarden]

http://dev.mysql.com/doc/refman/4.1/...functions.html

FIND_IN_SET(str,strlist)

Returns a value in the range of 1 to N if the string str is in the string list strlist consisting of N substrings. A string list is a string composed of substrings separated by ‘,’ characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic. Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function does not work properly if the first argument contains a comma (‘,’) character.

mysql> SELECT FIND_IN_SET('b','a,b,c,d');
-> 2

PS Did you set "Is banned usergroup" for that usergroup?