So to review: You click and type to add text, and you double-click a shape to edit text. That's pretty straightforward. But what if you need to edit text for a shape that's in a group? You could try to use the same methods, although it might not work all the time because of the way some groups are configured.
Here's a good method for adding and editing text for grouped shapes:
Click the group of shapes with the Pointer Tool
Click again to select one of the shapes in the group.
Click the Text Tool
Visio will zoom in on the shape text. Add or change the text as you see fit. (Remember to click an empty area of the page to zoom back to the level you were at originally.)