Enable MySQL trigger for all users in mysql
If you think of the definer as the user that the trigger runs as then it needs the right grants to do whatever the triggers action is.

But it sounds like what you want is for other users to be able to trigger the trigger. In this case all of the users that you need to be able to invoke the trigger need the trigger privilege.

See more info here: http://dev.mysql.com/doc/refman/5.7/en/privileges-provided.html#priv_trigger

But basically for each user you need to grant trigger.