I was afraid there was no easy way ..
Essentially what I like doing is, for example, to have a little herd of fleet ships as my control group #1, and I will often put some of the "special" fighter type ships with them (electric bombers, medic frigates, spire railclusters, that sort of stuff) but those light ships tend to get whacked frequently during combat, much more so than the fleet ships.
So what I'd like is to have MSDs build replacement ships during the fight (I usually bring a few MSDs into a planet attack, and just park them near the wormhole once I murder any tachyon posts) - and I want those replacements to be in control group 1, so they just get grabbed into any new orders I issue without having to tediously select them and add them. Putting the MSDs in group 1 doesn't work because then they'll move around with my fleet, so they can't repair and will just get gradually beaten up and killed.
Could this be a feature request?
Change things so all constructors - space docks, MSDs, etc, have an option for the control group their produced units go into, rather than having their the units get assigned to the constructor's control group? There'd be no loss of functionality and no invasive UI requirements.
Right now if I decide I want another space dock in my "1" group, I have to consolidate & select all of the control group units, then grab all the constructors together and control-group them again.
With selectable output groups, I could just build a new constructor and pick "1" on it, and it'll just work automagically.