Is your feature request related to a problem? Please describe.
It is common to duplicate dynamic groups for class auras, boss mods etc. across multiple classes/specs/encounters. Changing becomes cumbersome since it needs to be changed multiple times and editing multiple groups simultaneously isn't possible.

Describe the solution you'd like
Conceptually just using another dynamic group's display, grow (and sort?) functions. Perhaps you'd register the target group as an anchor group or some such.

Why not meld the groups into one? Raid specific auras are unlikely to be active (&thus affect layout) in a dungeon.

you might want to have different grow directions for specific set of spells handled by groups

That sounds like a lot of complexity for not much gain. There are after all ways to share code via the common environment.

@InfusOnWoW could you point me to PR that added group in group support ? I wanna check it out how complex it would actually be for my own sake

That's a complicated story, there was a PR but it was incomplete and buggy. So I took apart and over a year integrated various bits while ensuring that everything continued to work. The last change that actually enabled nested groups, dfba73f and 68aad84 are not that big.

The PR #2965 did cotain a buggy implementation of nested dynamic groups, but due to its bug it wasn't merged.

I did promise that I eventually implement that, but since I'm not 100% sure how to best implement it, it's probably a major project of which I manage around one per year.