If you have an idea for a new group on groups.drupal.org, first make sure it's really new by searching existing groups. Someone may have already created a group similar to your idea that you could join.
New groups are not shown in searches, making them hard to find for anyone you haven't directly invited. To make your new group show up in searches, you can request review and approval by creating a new issue in the groups.drupal.org project. Groups.drupal.org moderators will review your proposal and may ask questions, so be sure to check back on this issue until it is resolved.