If friends want to remain on friendly terms they should not share the cost of a single timeshare. I have always wanted a second timeshare, but the cost was too much. One of my friends wanted to go in with me, but she already had one with another friend. They hardly ever used it, but the friend would not sell. They pay maintenance each year and it just sits empty. This did not set well with me.
Solution -- I bought an EOY (odd year). She bought an EOY (even year). She is always welcome to come during my year and I am welcome to come during hers. Each of us will help with the other's maintenance fees ( a minimal sum) if we go, but are not required if we do not go. We invite whomever else we want during our weeks. We agreed to let each other know what our plans are well in advance so that the other can plan on how many others will make up the group.
This should work well. If either of our financial circumstances should change, the other is not responsible for any payments. If one decides not to use her week, the worst is that the other will not get to go that year, but can count on going when her year comes around.
Whatever you decide to do, I hope it works out well for you.
JMM