Basically, you'll set up a segment that will look for people:
1. Audience Groups - "In" GROUP A 2. Response Data - "Opened" - "Mailing Name"
You can layer in additional groups if you'd like or make that Clicks instead of opens. Just make sure that if you add in multiple groups, you want to make sure the logic is set to "ANY" (which uses OR) instead of "ALL" (which uses AND). Leaving the logic set to ALL will only bring back people who are in BOTH of the groups.