If It involves only two layers of father son relationship, I am check they category.parent, I haved implemented the function, I wonder if this method is advisable.
I need plugin for send meesege to all users
that Admin can to send meseege to all users and mention a group of all useres
I don't think that there is one. But you could make one yourself, of course.
You could have the same effect, I think, by just posting in the group, though. Once you want to get a public message to everyone on the forum, the forum is the best mechanism for that.