DENOG is the short form of 'German Network Operators Group' and itself describes as a forum for and about german network operators. It's similar to NANOG and SwiNOG.
As a forum, DENOG has no memberships and no official presence - whoever is interested can participate, there are numerous ways:
DENOG-Meetings are annual events to meet, talk and chat about all aspects of their work or to give lightning talks to network related topics.
Meetings are once a year and will be organized by DENOG members in Cooperation with a company who acts as a host. There is a meeting fee, however this is needed to support the official program, meeting venue; mainly sponsors cover most of the costs.
More information about the DENOG-Meetings can be found there.