You can't just "add" people to a contract. You are buying joint real estate with another adult. There are a lot of reasons not to do this. Divorce, bankruptcy, lawsuits, I can keep going. I would never do this with an adult I was not married to.
If you hold title as different people or configurations of people they are different membership numbers. There have been some stories of
DVC correcting Mickey Mouse and Mickey A. Mouse. But Mickey Mouse and Goofy will always be a different member number than Mickey and Minnie. Different member numbers are like different people as far as DVC is concerned.
The other option is to buy as something like an LLC or trust. Lots of people do this as part of their estate planning. If this is part of your estate plan, maybe smart to get local legal advice and do this right from the jump.