So, I'll post my 2 cents. Our additional family members have been added by purchasing an additional contract (as several have noted above). It's been really nice. They are not financially responsible for the whole kit and kaboodle, only the small contract they have been added to. One common name (on the original deed and the additional contract) will allow the contract to be added to the existing membership.
I bet we saved close to $1000 this month alone. Couple ski trips to Bentley Brook (I have sisters that live in the area, they and their kids are regulars there). Maybe 4 stays at Bentley Brook. Son went to New Orleans Mardi Gras week and stayed at La Belle Maison. Brother stayed at Ocean Walk Daytona 500 weekend. 3 nephews got together in Colorado for a ski week and stayed 4 nights in Avon. It was an unusual month, but there have been other times we've had heavy usage like this (as I often lament I am the only one not having fun somewhere, woe is me).
A not often mentioned advantage to being owners on the account is the extra flexibility it gives us to book. In New Orleans, I pieced things together. 2 nights + 2 more nights, etc, as I found them. I can do that with an owner. If the room types are the same, they stay in the same room. As an owner, there are no guest fees. If not an owner, it's cost prohibitive to piece things together as each piece needs a GC. In Avon, they had a 2 BR Pres 2 nights and a 1 BR Pres 1 night. Again, much more doable for an owner. (And often, if you are willing to voluntarily downgrade (in this case to a 1 BR Pres) they will keep in you the same room for all of your stay (I have no idea if my nephews did that)). It's nice for you (not to move) and saves on housekeeping, the resorts don't have to clean. A win-win for the owner and the resort.
Also worth noting, as people so often complain about not being able to book... we had someone staying in Daytona Beach (3 BR, BTW) during the Daytona 500, New Orleans during Mardi Gras, and skiing in Colorado President's week. (New Orleans and Avon were presidential units). All booked last minute.
We do have a lot of fun stories, we've certainly used and enjoyed the ownership. My daughter stayed at the Grand Desert in a 4BR Presidential with some friends, she was probably mid 20's at the time. Rode the elevator multiple times with the sales people on tours. Several times they were asked if they were they going to the correct (presidential) floor. So they enjoyed that. My son went to New Orleans, not for Mardi Gras, but for a wedding. One of his best friends 'eloped' and invited just a few people. The friend lives in New Orleans. Lundi Monday (hope I am saying that right) is a day of spontaneity? So they wandered about til they felt like getting married, and got married in Jackson Square. Son had a presidential suite at La Belle Maison, so that is where they ended up for the reception. I think 10 or so of them. He texted me to be sure to leave a good review for the front desk staff, he just couldn't believe how awesome they were. We have had countless family reunions -- Daytona Beach, Orlando (Bonnet Creek), Myrtle Beach, Tennessee, Oceanside, CA, Anaheim, CA (we did have Hawaii booked this year but had to cancel - hoping for next year). Having 5 or 6 rooms for the reunions, it sure is nice to be owners.
All the words of caution, taking on financial responsibility, families sharing, must be taken into consideration. Everyone's situation is different. For us, this is working, things will change over time. I predict some day one of my nephews will take over managing it, I don't expect my kids to (one of three is not an owner, she said no thanks, I'll pay the guest fee). I did my best to lay out the pros and cons, advantages and risks, and each family member made their own decision. The way it works today, whoever uses it pays the maintenance fees (at the going rate) for the points they use. Excess points are rented.