If you are using 2007 (and maybe 2010), first collapse your schedule to outline level 1. Then expand sub-groups (not levels) until you find the sub-group(s) you want to show. Expand it (them). At this point all the tasks for that (those) sub-group(s) is showing and along with the other summary lines. Now hit the auto filter for Critical. Only the critical path tasks in that (those) sub-groups will show (yes, you do have the summary lines from the other sub-groups showing – I’m not sure you can show just one [or selected] sub-groups without the other summary lines).