We have a pretty standard process for developing agendas for meeting like this:
1) We work with co-organizers like you two to articulate 3-5 "event outcomes" or "outputs", articulated in the language of those who will participate in the event.
Examples might include strategic planning assets (visioning statements, strategic and/or tactical goals, draft campaign, program and or product plans, partnering plans and frameworks, etc), organizational stock-taking (e.g. SWOT-type outputs), financial planning assets (budgets, fundraising goals and tactics, revenue diversification, etc), community strategies, governance designs etc.
2) We invite input from confirmed participants, asking them both whether the stated objectives sound like the right ones, while also inquiring about what is going to make their time feel most well-spent. This is normally done by introducing me to each participant via email, which I fork off to a 1-1 conversation, and then aggregate all the input back to a planning document we work from.
3) Based on all the above, we draft out a preliminary agenda, and think through who from the participant group could be well-suited to help facilitate different sessions or conversations within the agenda.
4) We then engage those facilitators, and work with them to think through simple and clear facilitation plans.
5) As the agenda comes into focus, we discuss documentation and capture, deciding when and how to capture meeting outputs.
6) At the event, we treat the agenda as dynamic, evolving the plan in real time based on both preliminary outcomes as well as participant feedback.
7) The agenda is "final" when the event is over :)