What must happen now to make the organization work like it should? What can you contribute to making these things happen in the next year (or other time frame)?
These questions can be stated in broadly strategic or nitty gritty tactical terms. What's essential is that they make sense to the people who will be invited to participate, and that those invited are actually the people who are ultimately needed to accomplish what is desired. First, draft something. Then test it out on any prospective participants and/or the sponsors or leadership group, inviting minor revisions or complete rewriting, until it "works."
What must your invitees create and do now, based on current conditions, data, news, priorities, resources, needs and all the rest? What is the future of "purpose"? What are the Issues and Opportunities for "desired outcome"? How Can We "succeed" Together Now?
Think of your invitation as a story, or a letter to friends and colleagues. Start it with "Once upon a time..." or "As you've no doubt already heard...", if you like. "Please join us! ...let's go! ...see you there!"
...and good luck!