Currently, Ulysses doesn’t include built-in collaboration features. Since iCloud is used to sync the library, sharing or editing content together isn’t supported. iCloud syncs data per Apple Account and doesn’t allow multiple people to access the same library from different accounts.

However, there is a workaround if the person you’d like to share sheets with also uses Ulysses. In this case, you can share files through third-party cloud services such as Dropbox, using External Folders in Ulysses. Here you can learn how to connect Dropbox to Ulysses. 

Alternatively, you can share an iCloud Drive folder and then add this folder to the External Folders section in Ulysses. Learn more about using External Folders here.