Get notification message in groups
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.