The posters here have made great points about why coordinating a purchase with family members is probably not a good idea.
That said, however, my family did buy a membership together in 1996, and it is still working well for us. There are 4 parties (my parents, myself, and my two siblings). We purchased 220 points total, which was enough for each family to use 55 per year (or to borrow/bank in other ways). At that point we could do 5 weeknights in an OKW studio during Magic season. We are all on the deed, and so all have full member benefits. We have used the points in all sorts of ways, including combining them for a Grand Villa once.
Since joining, my brother, sister and I have each added on 55 additional points. These appear as add-ons to the original contract. (We all had to sign for each add-on.)
Communication is essential, as all rules concerning banking/borrowing, etc. must be followed as a group. We end up doing a lot of banking and borrowing just within the membership (if someone wants to skip a year, another family often ends up using those points). I have become the keeper of the points, so that no one does anything without consulting me so that it all makes sense within our own membership and with the club rules. Confusing at times, but so far, so good. We also must coordinate dues payments so that all pay their fair share.
So, I probably would not recommend this as a way to purchase, because I can imagine how it could go very wrong, but I wanted to let you know that such arrangements do exist. For us it was a way to cut the costs of
Disney vacations, and to be able to stay in nicer rooms.
Good luck with your planning and purchase(s). I hope your family will enjoy their membership(s) as much as mine has done.