Identifies whether the user belongs to the specified group.
GROUP_MEMBER ( group-name-string-expression [ , user-name-string-expression ] )
group-name-string-expression Identifies the group to be considered.
user-name-string-expression Identifies the user to be considered. If not supplied, then the current user name is assumed.
0 Returns 0 if the group does not exist, if the user does not exist, or if the user does not belong to the specified group.
1 Returns an integer other than 0 if the user is a member of the specified group.