We're happy to announce that we've rolled out group membership options on Drupalize.me. We even created two different approaches, so that you can pick which one works best for your group. The first option will let you buy a set number of accounts for your group (see basic pricing information in our FAQ about groups) and the second gives you unlimited accounts when accessed from a particular range of IP addresses. Here is a brief overview of the two choices and how they work.
