Get notification message in groups I created groups using Group module.

One group is a job poster(Group-A) and the other is a business owner(Group-B).

Group-B business is all about plumbing.

When Group-A posted a job about plumbing, Group-B should be able to get notified about the post as this is related to his business.

So, I use message stack as a method, but I still didn't get in the right way.

There is a message_notify module example that displays details in a page about a certain actions.

So what I need is that a group should get a notification message whenever a related job post is created.

Thanks!
What you need is to use the Message Subscribe module, also in the Message Stack. This will allow you to subscribe users to a particular entity, e.g. - your job post.

You need to be able to link your "plumbing" (taxonomy tag, or direct entity ref) to Plumbing Group. You need to use Rules if the functionality exists, or a custom module to hook into the submission of a new job posting with the reference or tag to pointing to plumbing group. If this tag exists, then subscribe all users in the Plumbing group to that new job posting and issue a notification using Message Notify.