I've never participated in one (too slow!) but my understanding is that it's merely a group of riders doing a Permanent or a DIY simultaneously. Each rider follows the same procedure for the Permanent or DIY as they would if they were riding solo i.e. each person enters/gets their own Brevet, etc. as normal. They just agree informally amongst themselves to synchronise their rides. It may not necessarily mean sticking together all the way around, or even starting together, as some riders might start at different points.
Nothing official about it; it all fits in the existing rules and procedures.
The interesting point, perhaps, is that with the advent of GPS validation, the creation of DIY rides is easier, not least because there's no need to hunt out controls which deliver usable till receipts, nor to create routesheets; one person can create the ride and pass copies of the GPS track/route to the others (always assuming everyone uses a GPS of course, or agrees always to stick together on the ride.)