Tim Hardwick, writing for MacRumors:
On the face of it, sharing the location of an AirTag via Apple’s Family Sharing feature should be a no-brainer, since individual members in a Family Sharing group can use the Find My app to see the location of other family members’ Apple devices, including iPhones, iPads, Macs, AirPods, and Apple Watches.
Given those Family Sharing privileges, many users have understandably assumed that an AirTag attached to an item that’s regularly used by more than one person in the household (a set of keys, for example) could be tracked by multiple family members within Find My.
Unfortunately, however, AirTags don’t work like other Apple devices in Find My, and several users have taken to the MacRumors forums, Reddit, and Apple’s community support site to share their displeasure after learning about the Family Sharing limitation
This does seem like an odd omission. And I didn’t even realize this was the case until I read the aforelinked piece about it. Hopefully it’s just a case of an early product and this particular feature hasn’t been implemented yet.