MYW tickets that are purchased in a single transaction can be used interchangeably by anyone in the original group of users, for whatever options you have purchased, because the encrypted info on each of them is set for the entire series of serial numbers included in that purchase, rather than a single number.
The biometric info that is taken at first use is linked to whatever serial number or range of serial numbers that are encrypted onto the MYW pass. If the MYW pass has the serial numbers of several MYW passes encrypted on it, then any MYW pass in that series of "grouped" MYW passes can be used by anyone whose biometrics are part of the group. What this means is that if someone who was in the first group does not return on a later visit, the unused remainder of the pass could be used by someone who WAS part of the original group from the first visit. So, if family A buys 4 MYW passes (w/ no expiration) for trip1 in June, for DW, DH, GP1 and GP2, and then DW and DH go back in July for trip2 without GP1&2, DH & DW can use the remaining days on the passes originally used by GP1&2. If they decide to bring GP3 & GP4 on trip2 in July, then GP3&4 are going to have to buy new tickets, as they were not in the group in June.
Note that the "grouping" of the tickets happens at purchase, not at first use, even though the biometrics are not added until first use. As far as I've been able to determine, there is no procedure set up to later "group" MYW tickets that were purchased in separate transactions.