I've started a few meetups and a format that works well is the following:

- have one company as the sponsor, providing venue, food and one technical talk - this ensures that the organizers don't need to deal with money which is a big admin headache - have around 3 talks total, but keep them short, to around 20 minutes + 5 minutes for Q&A - start the event 15-30 mins late to allow for people to show up late and giving those that turned up early the chance to talk to each other - make sure you end quite early, leaving time for people to mingle afterwards

We also built our own Meetup.com alternative called Meetabit (https://meetabit.com, see my other comment to this post) and to promote it wrote a few blog posts which go into more detail, which you can find here: https://blog.toughbyte.com/tagged/meetup