Currently it is a little complicated to create a group:
- Create a page for your group. It is a good policy to manage your group in the XWiki space. Example: XWiki.MyGroup
- Go to this page
- Use "edit objects" to edit the objects of this page
- In the Add Object field choose "XWiki.XWikiGroups"
- Repeat the "Add Object" operation for each user you want to add in the group
- In the member field of each object add the full name of your user. Example: XWiki.LudovicDubost
- Eventually protect your group from editing using a right.
- Your group is usable in the groups fields of rights using "XWiki.MyGroup"