Say I have Marines, Wraiths and Siege Tanks selected in one group binded as group 1. Now what's the easiest way of i.e. selecting all Wraiths from the group so can I can quickly cloak them. I know I could assign them to different group but I don't want to do that.
You can ctrl+click the unit in the hud. That will select the same type of unit in the group even if they are not visible on screen.
There is a certain key combination to cycle through subgroups in a group. Something with TAB.
Alternatively you could double-click on one unit which selects all units of the same type.