Please, do not notify users or groups in your first post of a topic or conversation (that post is what you have to write for creating a topic)
It’s a waste of emails and notifications because we usually read every topic, so if we can help we will reply you; and also, you are forcing them indirectly to answer you even if they don’t know how to solve that problem, or making other’s reply less important if they haven’t been mentioned
But you can notify someone in a reply he or she has already participate on it, or if you want to mention a topic that he made for helping