Starcraft – How to clear unit selection in Starcraft 2

I don't think there is a way to do exactly what you describe. If I am wrong, I would love to know about it!

The second best thing I can find is to set a waypoint to follow a unit in your main attack group. To do this, select your production buildings, find a unit that isn't likely to die anytime soon, and right click that unit. New units being produced will follow your main group, and will attack nearby enemies and such.

However, they still won't be in the actual control group, so you can't easily use their spells and whatnot. In order to set that up, the easiest way I've found is to select the units as they're walking toward your group (it's good to listen for "construction finished" sound bites and hit Space) and then press Shift+3 to include them in the original group.

Two things to watch out for:

1. Units heading toward waypoints are using the "Move" command and will not attack stuff en route.
2. If the unit you are following is destroyed, the waypoint will end up wherever it was when it was destroyed. If this is right at the front of the enemy base, that's bad.

You can do multiple control groups.

Group 1 = all of them
Group 2 = marines
Group 3 = marauders
Group 4 = tank
Group 5 = banshees

Selecting your other control groups does not lose your full control group. Very useful for air.