Yes! Before you parent the control to the group, parent the group to what the same parent the control is in, so the original rig hierarchy doesn't change.
Also it would probably be even better, if instead of constraining the group to the horn, you constrain the group to a locator at the same position and then move the locator to the horn and parent it under it.
It's all a bit hard to explain via text. So I'll give you the script I normally use to do this.
Just select your horn cluster and then your hand ctrl and run this script in a python tab in your script editor.
#select parent first then child
import maya.cmds as mc
def constCTRLtoObj(parentObj = None, childObj = None ):
""" creates animation save constraint from parent (selected first or kwarg parentObj) to child (selected second or kwarg childObj)"""
#defining parent and child
sel = mc.ls(selection = True) or [parentObj,childObj]
#getting parent above child
childParent = mc.listRelatives(sel[1], parent = True)[0]
#getting position matrices
cpma = mc.xform(childParent, m = True, q = True, ws = True)
pma = mc.xform(sel[0], m = True, q = True, ws = True)
#creating separation objects
grp = mc.group(name = '{0}_const_GRP'.format(sel[1]), em = True)
loc = mc.spaceLocator(name = '{0}_offset_LOC'.format(sel[1]))
#moving into position for initial constraint
mc.xform(grp, m = cpma, ws = True)
mc.xform(loc[0], m = cpma, ws = True)
#parent child object to separation grp
mc.parent(grp, childParent)
mc.parent(sel[1], grp)
#constraining to separation locator
mc.parentConstraint(loc[0], grp, mo = False)
#moving and parenting Locator
mc.xform(loc[0], m = pma, ws = True)
mc.parent(loc[0],sel[0])
constCTRLtoObj()
I hope this helps!
