What are the GLAMs reporting obligations when reporting a qualifying resale when the purchase is made from a collective? (e.g A group of artists who work collectively or under an umbrella ‘collective’ organization).

The same reporting rules apply. Assuming it is a qualifying resale (i.e. not a first sale), the responsible party must report the sale under section 21.  That must include information about the name of the artist (or artists) if known.