[Greg Titus]

By the way, this ability got added to OmniFocus for iPhone 1.0.2 which is available now.

The way you'd make the action group on the phone is to create your "Tidy Power Cables" (the group to be) the same way as you would a normal action, and the same with "Investigate fittings" (an action you want inside the group). Then from the "Investigate fittings" details screen, tap on the reorganize icon in the toolbar (the arrow and folder icon on the right side). Choose to move that action inside "Tidy Power Cables". That will make "Tidy Power Cables" into an action group.