As I’ve realized that it was triggering upon macros in group getting available, more than upon group activation, I've changed the threads title, and rewritten this post better to reflect this. A Group Activation trigger could also be useful, but Group Getting Available trigger would bring with it more unique usability.
I’ve many times found myself thinking, and also mentioned here before, how a Group Activation trigger could be very useful; triggering a macro when a certain Macro group Activates, or Gets Available. Oftentimes it would be used to trigger a macro within the group having become available, so an option for 'When THIS Macro Group Gets Available' would make allot of sense. But this trigger would ideally also have options for selecting the macro group from a list of all groups (similar to the how it’s done in the Activate/Deactivate Macro Group acton). Different trigger settings for when the selected (or THIS) group "Gets Available" and "Unavailable" (as well as "Activates" and "Deactivates") would be great.
In the context of this trigger I believe I view change of group availability status as being the same as activation/deactivation. So maybe it could be combined into the same trigger/setting, and would trigger just the same if the group was activated by a hot key or an a Activate Macro Group action, as when it becomes available by bringing a certain application or window to the front, (or the like). The %TriggerValue% should ideally differentiate between the two though, as well containing details about the event activating, deactivating or altering the groups availability status.
If there'd be an option for 'When THIS Macro Group Deactivates' I believe it could make sense for the macro to be triggered and ran (once), even if it’s group at this point is in fact deactivated, but I see how this specific behaviour is somewhat strange, and I am not at all sure about this one. I am also not sure if 'When THIS Macro Group Get Unavailable' would here pose the same problems as Deactivates, maybe not?
A somewhat mirror-related feature to the one above that I’d also find very useful, is a Group Availability option (in the Macro Group settings) for "while Macro with UUID is executing". Ideally the field for adding the UUID should accept several UUIDs separated by commas, making it possible to add more macros than one. I guess it could also be very useful if there where options for "Available when a Macro with name contains/matching is executing". When thinking about it now it could also make allot of sense for there to be an option for "Available while any macro in group(s) with UUID/title is executing", making it possible to add availability for several macros through adding them into a certain group or groups.
) -- is there an actual problem with macros triggering with such frequency? Sure, lots of "unnecessary" triggers recorded in the Engine log, but the resource cost is next to nothing compared to everything else going on.
Of course, we don't want the macro to be triggered more than once, so it ends with an action to disable its macro group. Right before that, it enables the complementary macro group – which can re-enable the first group.
If anyone wants to experiment along these lines: heed the warnings on the Wiki page about setting a short interval for the 
