You can create and adjust ranks for your group by clicking Group Manager > Group Ranks.
This screen is blank if you have set no ranks. You'll see a "New Rank" button at the top right.
Click New Rank and you'll see a form asking for the name of the rank and the number of points. You can set the number of points as a range, using a hyphen, as in 0-100.
Once ranks are set, the Group Manager > Group Ranks page will show you the current names of ranks and their point ranges. This same list is also visible by clicking your current rank at the top of the screen.