To start, once every two months is probably a good formula. This will allow plenty of time between meetings to ensure that you have a facility secured and can select a date that works best for most members. After your first 2-3 meetings, you may want to consider increasing the frequency, based on member feedback and attendance numbers.
The format of the meet up can vary. Many of the groups kicked off with the organizer doing a presentation on "APEX 5 New Features", for instance. Other groups had a more collaborative format, where members discussed issues/problems that they have with APEX and other members tried to offer advice.
It may take a couple of meetings before others are willing to step up and present, so as an organizer, be prepared to have some content to offer. Offering an “open mic night” format - where each session is 5-15 minutes of demo only - is a good way to break the ice and encourage people to participate. Also, if you're near an Oracle office, reach out to them and see if they would be willing to send someone to the meet up. Even if the topic is more closely related to the database or other technologies, the group can try to relate how that would help them with their APEX projects as part of the discussion.