Hi Rachel, the Boston R User Group is huge! So I would expect that you and your co-organizers are the experts! I organize a much much smaller R User Group in Costa Rica. But you ask, and maybe the lessons we learned so far, after 26 meetups, are useful for you.
Looking at the top 5 events with most attendance were:
- Creating Dashboards with Shiny
- An industry use case (someone from a large corporation talking about how they use R)
- Timeseries in R
- Applications and visualizations in R for online casinos
- Introduccion to graphs with iGraph
Speakers come from my network. Even though there are more than 400 people registered on meetup, and I regularly ask via email for proposals and ideas for talks, I never get a response. So I ask friends, colleagues and customers and that usually works out well. We hold the meetings every month, so I do have to pull in some favours, but usually it is an enjoyable experience for the speaker, and it certainly is for me, as I always learn something new.
Our experience is that the only way to get an accurate headcount of who will attend, is by raising a fee. The R Use Group Meetings are free, and they are online, so that people do not have to travel to attend. Even so, there seems to be little relationship between the registrations and the amount of people showing up. It varies a lot. We also sponsor and organize community events (through Data Latam), and there we always ask for a fee to cover expenses. As soon as people have to pay in advance the chance of them attending increases from low to near certain.
We usually have only one speaker. But this is mostly because of the format (online meetings). We do however try to have as lively a discussion as possible afterwards. A panel may work in a live setting though.
I hope this helps a little. Good luck with the organization!