Let's say there is an expense, and it's shared between a number of people. What do you call the person who has a share in that expense? Sharer?
If it is an ongoing expense (like for a gym-pass) "stakeholder" would be understood but stilted, since it stems from business-environments (and is mostly misused even there).
If it is a one-time deal (like for a meal) "contributor" might work. e.g. "She was a contributor to the birthday present, so she got to sign the card"