Don't call it a meeting. Sometimes, a word like workshop does the job, but it is overused and can mean different things to different people.
...you call it a 'decision-making session'.
This will stand out in people's email inboxes. It sounds final and it has an action attached. People in places of authority or responsibility either love, or are scared of decisions. Either way, you probably have their attention. It also sends a clear message to those people. They: are expected to make a decision, may feel they'll miss something important if they are not there, and are much more likely to open that email.