The suggested group MMSI number is out of the blue, since it's not registered or issued, as are individual numbers. It has to begin with a 0, but other than that, you can pick whatever you want. Thus, the date-based number.
As I understand the group function, anyone who has that number programmed in as their group MMSI is, by default, a member of the group. When any group member sends out a group DSC transmission, anyone else within range who has that same group number programmed in receives the call, and has their VHF switched to the designated channel. I don't know if the channel switch is automatic, or only upon the recipient's approval, on a call by call basis. Our's is set up to switch automatically (I think).
So, any individual vessel can have two MMSI numbers. The individual number is permanent, and the group number is out of the blue, and can be changed from time to time based on whatever group, if any, the individual wants to be a part of.
I've also set our VHF to automatically respond to any DSC calls to our individual MMSI requesting our positon.
So, if our boat name and MMSI is programmed into your VHF and chartplotter, a request for our position will result in a demand that our VHF supply our position, and you will automatically receive that data back on channel 70. The data (name, lat, long) will be displayed on your VHF and on your chartplotter, where you will see an icon with our vessel name, and be given the option of creating a waypoint at that location. PRETTY COOL.
I'm really looking forward to seeing how this all works out. There have been numerous occasions where I've gotten separated from another boat, and because of fog, heavy rain, or just general confusion, had a tough time locating them so we could get back together. Heavy fog, and heavy vessel traffic, makes it hard for me to sort out one target from all the others on the radar screen, and getting a VHF audio transmission of lat/long is only of marginal use, particularly if you're solo, and messing around too much with charts or the plotter distracts you from keeping a lookout. I'd really like to be able to send a position request, get the location info back, and have it appear on the chartplotter. There, I can make it a waypoint, which will automatically also appear on my radar screen, and I can quickly lay out a route to take me to the target. This would be so easy, you could even do a position update along the way.
Take care.. Tracy
Call either Carrie or me if you need anything. Have a safe and happy trip !