My reading of GHB 20.2.1 is that calling committee members is described, but there doesn't seem to be any indication about calling a committee leader (chair/etc). It does say that a Ward Council member is assigned to be in regular contact with the activity committee, so that seems to be who functions as the de facto activity committee leader.
That said, if a ward wants to create a custom calling for Ward Activities Committee Leader, nothing wrong with that. When I go to the LCR "Additional Callings" I don't see that calling listed as a standard calling available for us to select, so as suggested it would just be a custom calling.