We are allowed joint tenancys, and if one person decides to give up, the other named person can keep the plot, but if they are not named on the tenancy then they can not keep the plot, instead it goes to the next person on the list.
You can't add names, if you want a joint tenancy, you have to do it when you first get your plot, and for each person named you have to pay a £3.50/year admin fee.