I found the relevant canons at the vatican website. Canon 312 to 326 seems to cover it fairly well.
Can. 312 §1. The authority competent to erect public associations is:
1/ the Holy See for universal and international associations;
2/ the conference of bishops in its own territory for national associations, that is, those which from their founding are directed toward activity throughout the whole nation;
3/ the diocesan bishop in his own territory, but not a diocesan administrator, for diocesan associations, except, however, for those associations whose right of erection has been reserved to others by apostolic privilege.
§2. Written consent of the diocesan bishop is required for the valid erection of an association or section of an association in a diocese even if it is done by virtue of apostolic privilege. Nevertheless, the consent given by a diocesan bishop for the erection of a house of a religious institute is also valid for the erection in the same house or church attached to it of an association which is proper to that institute.
It seems pretty straightforward. No approval from the bishop, no order.
Thanks again for your help,