It is not uncommon for groups of communal organisms to share their senses. The Joined go farther and can share aspects of themselves among themselves. If, for example, 8 give up 1 foot of height, a ninth will become 8 feet taller. This applies to all aspects, the joined able to obtain photographic memory or become hypergendered, for instance. These changes are completely reversible and are only limited by how much the donating individuals are capable of giving up.
It is said wizards have replicated this ability in magical rings known as Bands of the Joined. Each person wearing one can partake in the same sharing as the joined normally do.