De-Selecting An Engineer From A Group

Once I have 2 or more engineers selected with a mouse drag I'll use them to build something faster - defenses, factory, etc. - or to support the building of units. However, when I want to split one of the engineers off I cannot figure out how to get just the one I click on to respond. When I click on one of the engineers to select it and give it commands, all of the engineers in that group are selected.

Am I missing a mouse-click technique or keypress? I've tried Shift-Click, Ctrl-Click, etc. with no seeming success.

And as that goes, I swear there are times I click the repeat queue and times I try to right click part of the terrain after selecting one or more units and the click just doesn't work. Perhaps it's me?

Thanks for any input!