Note: Groups are not always enabled
To start a group, go to the 'Community' tab and then 'Groups' (if groups are not enabled there will be no groups option). Then you can click 'Create a new Group'.
You will then be able to create a group with a group name, tagline, and logo. The image you upload for the logo will be cropped to be a square.
You will also need to decide if the group is open or closed. With an open group, anyone can join at any time. Also, anyone can see all posts and comments within the group even if they are not part of the group. In a private group, all posts and comments are private to only group members, and group members must be invited or approved by a group admin. Private groups still show up in the groups listing.
Once you have your settings as you want, just click 'Create Group' to create the group!
If you created a private group and want to invite your peers, see here.