I just added a group to the Notable Members page today and when searching for reminders how to do it this thread popped up.
For the current versions of XF, to add a new user group to the XF Notable Members page:
Create the new user group (ACP -> Groups & Permissions -> User Groups -> Add User Group) /admin.php?user-groups/
Add a new member statistic (ACP -> Users -> Member Statistics -> Add Member Stat) /admin.php?member-stats/
"Title" is required - This is the name you'll see on the Notable Members age.
"Member stat key" is required - This is the unique internal name of the member stat and is not shown publicly.
Change "User Criteria" from the default of "All Valid Users" to "Searcher Criteria"
Assuming most folks are following XF best practices of using secondary groups, in the "Secondary User Groups" selection choose the name of the new user group you created.
Everything else is optional and you can play around the settings to tweak them if needed but you may want to change the "Display Order" value near the bottom of the screen if you want the new group to appear in a certain order on the Notable Members page.