Ranks are a label and not in any way connected to user group permissions. You can, however, make a user group called, for example "Moderators" and place the moderators in that group - and set each user's rank to "Moderator"

User groups control permissions for access within the website, ranks are simply a label that can be assigned to any user. When users are put in certain groups, it maybe useful to give them a rank or identifier if they have a certain role in the management of the website.