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.