Presumably not true for `BasicGroup` when `CHILDREN_AS_MEMBERS` is set?
Perhaps could word as "does NOT by default add it...".
Or probably better we deprecate `BasicGroup.CHILDREN_AS_MEMBERS` as it is only used in tests in core brooklyn (and can instead call `addMemberChild`).

I don't follow how this relates to CassandraFrabric (based on commit description). The `addMemberChild` isn't called from anywhere that I can see.