Are you sure about this? I was under the impression that you could always have at least 5 people at a MM, even if you were in a room that only held 4 people (or, I suppose 2). The MM rules state…
Party Size Limits
A Member may register themselves and up to 4 additional Guests per Membership (for a total of 5)—or up to the number of Guests on their Resort reservation, so long as the Resort reservation was updated with the correct Guest information before the Resort reservation/modification deadline.
This seems to imply that you can always register for 5 and if you have a 2BR or GV, you could have more if they were on the reservation.
I’m really not sure, but I do know they’ve never checked for names on the reservation. I just need to show my Blue Card/ID/Magic Band and we’re all in.
I mention and ask this because I’m presently in a room for 4 but will probably still try for 5.