TalentCards allows you to reset a group code you have shared with specific users to prevent any unauthorized access by other users who might get a hold of that code.
To do that, go to the Users page (1) and click on the arrow next to "Add user" button to select "Invite Users" (2).
Go to the "By group code" (3) tab and click on "Reset group code" (4):
After resetting the group code, no user is allowed to sign in with the previous one. The change doesn't affect users that have already signed in with the old code unless they somehow delete the app and need to re-install it. In that case, you have to send them the new code.