After you create and add apps to a group, you can edit the group’s properties from the Game Center groups page. You follow the same steps to create and manage group leaderboards, leaderboard sets, combined leaderboards, and achievements as you would for a non-grouped app.
From My Apps, navigate to the ellipsis menu (...) at the top left of the page.
Click Game Center Groups.
After the Game Center Groups page opens, select the group you want to edit in the Manage Existing Groups section.
For more information, visit Group properties.