Create A Trigger That Adds Corresponding Records Into MessageRecipient

Relevant tables:

create table messages
(
msgID number(10),
fromID number(10),
message varchar2(1024),
ToUserID number(10) default NULL,
ToGroupID number(10) default NULL,
dateSent date,
constraint messages_pk primary key(msgID),
constraint messages_fk1 foreign key(fromID) references profile(userID) on delete cascade,
constraint messages_fk2 foreign key(ToUserID) references profile(userID) on delete cascade
);
[code]....

I need to create a trigger that adds corresponding records into messageRecipient when a message is sent to a group. So, for example, the sql statement

insert into messages values(1, 2, 'message sent to group 3 from user 2', NULL, 3, currentdate);

should add records of the form (1, userID) into messageRecipient for every (3, userID) pair in groupMembership. Here are two triggers I've tried, neither of which works.

create or replace trigger SendGroupMessage
after insert on messages
FOR EACH ROW
when (new.ToUserID is NULL)
BEGIN
insert into messageRecipient(msgID,UserID)
[code]....