This is surprisingly simple. Just go to theicarusproject.net/node/add/group. You can probably make sense of the form without these instructions, but just in case...
Group name
Enter the group name (try not to make it too long. For instance "The Minneapolis Icarus Local Group" could be "Minneapolis Local Group" or "The Minneapolis Group"). The longer your group name, the more likely you are to break your homepage layout (but don't despair! you can always edit it later).
Description
Enter a brief description of the group, it will be visible on https://site.icarusprojectarchive.org/og
Welcome message
Enter a longer description, mission statement or welcome message. This will appear at the top of your group homepage, above any group blog entries.
Membership requests
"Moderated" means a user has to ask for membership, then you, the group administrator, will get an email where you can either accept or reject them. "Invite only" means the user cannot ask for membership, you have to ask them. Finally, "Closed" means you control everyone's memberships exclusively. I would recommend going with moderated, if tons of random people keep bugging you for membership you can always switch it to 'Invite only'. Only check off the last one if you really, really don't want any new members.
List in Groups Directory
If you want your group to be sorta secret you can uncheck this and users won't be able to find it via https://site.icarusprojectarchive.org/og
Private group
If you want to be uber-secretive, select this one. Your group homepage will give people an 'Access Denied' message if they aren't a member. Keep in mind though that nothing's ever hidden from us administrators.