When you use the Show in Groups feature, you can adjust how the groups appear in your mail folders so that you can view only the group headers or all the e-mail messages in the group. To expand or collapse groups, point to Expand/Collapse Groups on the View menu, and then click the option that you want. Or, you can use the keyboard shortcuts to expand or to collapse groups, as follows:
To expand or to collapse all the groups at the same time, use CTRL + (plus sign) and CTRL - (minus sign). Use the CTRL + to expand the groups. Use CTRL - to collapse the groups. You must use the + and the - keys on your number pad for these keyboard shortcuts to work.
To expand or to collapse a specific group that you are currently viewing, use the LEFT and the RIGHT ARROW keys. Use the LEFT ARROW key to collapse a group. Use the RIGHT ARROW key to expand a group
~ Arun