Family Sharing
Can you share selected items on a Mac
(for instance Financial Info.) without sharing other info. like instant messages and emails?
Mac mini 2018 or later
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It would be more correct to think of Family Sharing groups as a way of sharing purchased items and subscriptions. Apple has the page What is Family Sharing? - Apple Support which explains it more.
What you are asking about is file sharing, which you can do without a sharing group. In Use iCloud to share and collaborate on files and folders Apple discusses how to share or collaborate folders and documents.
In both of these situations though you don't share Messages or emails - these remain the province of the Apple ID/iCloud account owing these services.
In one other sense can you share files on a Mac - with other user accounts on the same computer. In Share files with others who use your Mac Apple describes how to achieve this, though this is a bit 'old school' from the beginning of OS X when a family might have only one Mac in the household.